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1.
Axel K Schmitt Jan M Lindsay Shan de Silva Robert B Trumbull 《Journal of Volcanology and Geothermal Research》2003,120(1-2):43-53
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
2.
In the absence of convincing magnetic anomaly information, topographic profiles have been used to infer the tectonic history of the East Pacific Rise between 5°S and 20°S. Profiles projected at right angles to the rise crest show a sharp drop in elevation at roughly the same distance on either side of the crest. Profiles to the east of the rise also show a second topographic high at 95°W. Comparison of these profiles with empirical depth versus age curves for the North Pacific suggests that this rise, the Galapagos Rise, is the fossil East Pacific Rise which terminated close to 6 mybp by the spreading center jumping 900 km to the west. The extreme youthfulness of the present East Pacific Rise, the step structure of its flanks, and the similarity in age of the top of this step and the crest of the Galapagos Rise substantiate this interpretation. This jump coincided with a similar readjustment involving the Mathematicians ridge at 5° to 20°N and the opening of the Gulf of California. 相似文献
3.
The Tuxtla Volcanic Field (TVF) is located on the coast of the Gulf of Mexico in the southern part of the state of Veracruz, Mexico. Volcanism began about 7 my ago, in the Late Miocene, and continued to recent times with historical eruptions in ad 1664 and 1793. The oldest rocks occur as highly eroded remnants of lava flows in the area surrounding the historically active cone of San Martín Tuxtla. Between about 3 and 1 my ago, four large composite volcanoes were built in the eastern part of the area. Rocks from these structures are hydrothermally altered and covered with lateritic soils, and their northern slopes show extensive erosional dissection that has widened preexisting craters to form erosional calderas. The eastern volcanoes are composed of alkali basalts, hawaiites, mugearites, and benmoreites, with less common calc-alkaline basaltic andesites and andesites. In the western part of the area, San Martín Tuxtla Volcano and its over 250 satellite cinder cones and maars produced about 120 km3 of lava over the last 0.8 my. A ridge of flank cinder cones blocked drainage to the north to form Laguna Catemaco. Lavas erupted from San Martín and its flank vents are restricted to compositions between basanite and alkali basalt. The alignment of major volcanoes and flank vents along a N55°W trend suggests an extensional stress field in the crust with a minimum compressional stress orientation of N35° E. In total, about 800 km3 of lava has been erupted in the TVF in the last 7 my. This gives a magma output rate of about 0.1 km3/1000 year, a value smaller than most composite cones, but similar to cinder cone fields that occur in central Mexico. Individual eruptions over the last 5000 years had volumes on the order of 0.1km3, with average recurrence intervals of 600 years. The alkaline compositions of the TVF lavas contrast markedly with the calc-alkaline compositions erupted in the subduction-related Mexican Volcanic Belt to the west, leading previous workers to suggest that the TVF is not related to subduction. Trace-element signatures of TVF lavas indicate, however, that they are probably related to subduction. We suggest that the alkaline character of the TVF lavas is the result of low degrees of melting of a mantle source coupled with a stress regime that allows these small-volume melts to reach the surface in the TVF. 相似文献
4.
Suzanne Mahlburg Kay Beatriz Coira Gerhard Wörner Robert W. Kay Bradley S. Singer 《Bulletin of Volcanology》2011,73(10):1487-1511
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
5.
Ben S. Ellis M. J. Branney T. L. Barry D. Barfod I. Bindeman J. A. Wolff B. Bonnichsen 《Bulletin of Volcanology》2012,74(1):261-277
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
6.
Voluminous sheets of rhyolitic ignimbrites were crupted during Miocene time in a region of Central America that is underlain by a thick sequence of middle Paleozoic and older metamorphic and plutonic rocks. Strontium isotopic ratios of fifteen ignimbrites range from 0.7035 to as high as 0.7175. These values are markedly higher than those measured for cale-alkaline lavas of the same province, but overlap the range found in basement rocks that may have served as source rocks for anateetic magmas. This relationship is in contrast to that found in the western United States where siliceous ignimbrites are not significantly richer in radiogenic strontium than are the basalt erupted through the same basement series. Several possible models for the origin of the large volumes of siliceous magma are examined in terms of major-element and isotopic relations, experimental studies of phase relations, and the thermal requirements of melting or assimilating basement rocks. A mathematical model for the effects of assimilation in open and closed systems permits a comparison of predicted chemical and isotopic compositions with those observed and places limits on the plausibility of various schemes of contamination with or without concurrent crystal fractionation. None of the models is without its flaws. Recent suggestions that large volumes of siliceous magma may be derived from the mantle or lower crust explain certain aspects of the Central American ignimbrites very well if one postulates that the high strontium ratios resulted from contamination of the magma with radiogenic strontium released by the break-down of mieas in basement rocks through which the magmas rose. Such a model fails to explain the apparent restriction of large rhyolitic ignimbrite cruptions to areas underlain by thick continental crust. 相似文献
7.
Chris B. Folkes Shanaka L. de Silva Heather M. Wright Raymond A. F. Cas 《Bulletin of Volcanology》2011,73(10):1455-1486
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
8.
Anthony Hildenbrand Pedro Madureira Fernando Ornelas Marques Inês Cruz Bernard Henry Pedro Silva 《Earth and Planetary Science Letters》2008,273(3-4):289-298
New K/Ar dating and geochemical analyses have been carried out on the WNW–ESE elongated oceanic island of S. Jorge to reconstruct the volcanic evolution of a linear ridge developed close to the Azores triple junction. We show that S. Jorge sub-aerial construction encompasses the last 1.3 Myr, a time interval far much longer than previously reported. The early development of the ridge involved a sub-aerial building phase exposed in the southeast end of the island and now constrained between 1.32 ± 0.02 and 1.21 ± 0.02 Ma. Basic lavas from this older stage are alkaline and enriched in incompatible elements, reflecting partial melting of an enriched mantle source. At least three differentiation cycles from alkaline basalts to mugearites are documented within this stage. The successive episodes of magma rising, storage and evolution suggest an intermittent re-opening of the magma feeding system, possibly due to recurrent tensional or trans-tensional tectonic events. Present data show a gap in sub-aerial volcanism before a second main ongoing building phase starting at about 750 ka. Sub-aerial construction of the S. Jorge ridge migrated progressively towards the west, but involved several overlapping volcanic episodes constrained along the main WNW–ESE structural axis of the island. Mafic magmas erupted during the second phase have been also generated by partial melting of an enriched mantle source. Trace element data suggest, however, variable and lower degrees of partial melting of a shallower mantle domain, which is interpreted as an increasing control of lithospheric deformation on the genesis and extraction of primitive melts during the last 750 kyr. The multi-stage development of the S. Jorge volcanic ridge over the last 1.3 Myr has most likely been greatly influenced by regional tectonics, controlled by deformation along the diffuse boundary between the Nubian and the Eurasian plates, and the increasing effect of sea-floor spreading at the Mid-Atlantic Ridge. 相似文献
9.
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
10.
The melting curve of iron monosilicide, FeSi, has been determined in a laser-heated diamond anvil cell from 6 up to 70 GPa by direct visual observation of the continuous laser-speckle motion in the liquid state. At 12 GPa and 1700 K, a discontinuous change in the slope of the melting curve indicates the first-order phase transition between the ?-FeSi (B20) and the CsCl-type FeSi structures (B2). During the phase transition the coordination number of both, Fe and Si atoms, increases from 7 to 8. Above this pressure, the melting curve rises steeply but shows significant flattening at higher pressures. A comparison with the melting curve of Fe shows that both curves cross at 32 ± 3 GPa, FeSi having higher melting temperatures (about 100 K) at high pressures. 相似文献
11.
Major and trace element and Sr and Nd isotope data is presented from the Andagua valley scoria cone and lava field (15°32′ S 72°19′ W), Southern Peru in the northernmost part of the Central Volcanic Zone (CVZ). The rocks are all quite evolved in composition (SiO2 = 55–64 wt.%) and classify as benmoreites, latites and few mugearites and trachytes. Samples are characterized by high Na2O (4.2–5.2 wt.%), Sr (600–1300 ppm), Ba (800–1600 ppm). The main difference between the benmoreites and latites is in the Na2O content that reach the highest so far reported from CVZ for these SiO2 concentrations. The rocks are generally nearly aphyric but latites and trachytes are more porphyritic. Amphibole microphenocrysts generally are only present in latites and trachytes. The difference between benmoreite and latite samples is reflected in lower P2O5 and Zr content of the latite samples documenting the existence of two compositional different parental magma types. The investigated volcanic activity spans the Pleistocene to Recent with the historic activity concentrated in the area just south of Andagua. Combined relative stratigraphy, petrography and geochemistry define volcanic units and demonstrate that rocks from Chilcayoc Grande, Chilcayoc Chico 2, Jenchana, Sucna 1 and Chilcayoc Chico 1 represent the most recent volcanic activity. The main trend samples, each form a co-magmatic group resulting in sub-parallel trends in many variations diagrams. It is furthermore shown that these trends point towards calculated mixing lines relating the individual units through a binary mixing process, thus indicating a two stage evolution. In the case of Jenchana, Sucna 1 and Chilcayoc Chico 1, the samples define positive correlation trends in the Sr vs. Rb diagram that can be extrapolated back towards origo indicating nearly perfect incompatibility of Sr and Rb. This together with generally high Sr/Y (50–105) and low Y content (< 16 ppm) suggest lack of plagioclase fractionation and residual garnet in the source and is taken as evidence for relatively high pressure (lower crustal) origin of the mixing event. The amphibole bearing samples form individual co-magmatic groups that cannot be related to each other. This means that the amphibole bearing samples originates from different magmas. The lavas of the Ninamama group are comparable in age to the main trend samples but different in petrography and composition, why the two compositional different magmas must have existed within a small confined area within a limited time span. 相似文献
12.
High-pressure partial melting of garnet pyroxenite: possible mafic lithologies in the source of ocean island basalts 总被引:6,自引:0,他引:6
Many ocean island basalts (OIB) that have isotopic ratios indicative of recycled crustal components in their source are silica-undersaturated and unlike silicic liquids produced from partial melting of recycled mid-ocean ridge basalt (MORB). However, experiments on a silica-deficient garnet pyroxenite, MIX1G, at 2.0-2.5 GPa show that some pyroxenite partial melts are strongly silica-undersaturated [M.M. Hirschmann et al., Geology 31 (2003) 481-484]. These low-pressure liquids are plausible parents of alkalic OIB, except that they are too aluminous. We present new partial melting experiments on MIX1G between 3.0 and 7.5 GPa. Partial melts at 5.0 GPa have low SiO2 (<48 wt%), low Al2O3 (<12 wt%) and high CaO (>12 wt%) at moderate MgO (12-16 wt%), and are more similar to primitive OIB compositions than lower-pressure liquids of MIX1G or experimental partial melts of anhydrous or carbonated peridotite. Solidus temperatures at 5.0 and 7.5 GPa are 1625 and 1825°C, respectively, which are less than 50°C cooler than the anhydrous peridotite solidus. The liquidus temperature at 5.0 GPa is 1725°C, indicating a narrow melting interval (∼100°C). These melting relations suggest that OIB magmas can be produced by partial melting of a silica-deficient pyroxenite similar to MIX1G if its melting residue contains significant garnet and lacks olivine. Such silica-deficient pyroxenites could be produced by interaction between recycled subducted oceanic crust and mantle peridotite or could be remnants of ancient oceanic lower crust or delaminated lower continental crust. If such compositions are present in plumes ascending with potential temperatures of 1550°C, they will begin to melt at about 5.0 GPa and produce appropriate partial melts. However, such hot plumes may also generate partial melts of peridotite, which could dilute the pyroxenite-derived partial melts. 相似文献
13.
Potentially active volcanoes of Peru-Observations using Landsat Thematic Mapper and Space Shuttle imagery 总被引:1,自引:1,他引:0
Landsat Thematic Mapper (TM) images and Space Shuttle color photograph have been used to make a synoptic study of the volcanoes of southern Peru (14°–17° S), the northernmost portin of the Central Volcanic Zone (CVZ 14°–28° S) of the Andes. Apart from providing consistent coverage, the chief merit of the TM for this study has been the spatial resolution provided by the 30-m pixel size. The optimal 20-m resolution, variable lighting and viewing geometry, and stereo capability of the Shuttle photography provided an invaluable ancilliary data set. At the resolution available, subtle glacial-morphological features such as valley and terminal moraines can be confidently identified, and these features have been used to determine the relative ages of volcanoes. Volcanoes have been classified as potentially active if they have; (i) a well-preserved summit crater, (ii) pristine lava flow texture and morphology, (iii) flank lava flows with low albedo, and (iv) evidence of postglacial (<10 000 years) activity. Eight major volcanoes are postulated to be potentially active. Most are large, dominantly andesitic, composite cones with edifice heights of up to 2500 m; some of which threaten nearby settlements. One of them, Sabancaya, was active as recently as July 1988. Other, little-known, postglacial volcanic features include Huaynaputina, site of a major explosive rhyolitic eruption in 1600 a.d., and several fields of monogenetic scoria cones and lava flows. The active volcanic front is some 200 km east of the Peru-Chile trench, and the volcanoes lie on a trenchparallel trend oblique to the EW subduction. This narrow volcanic zone is thought to reflect the steep dip of the Nazca plate through the zone of magma generation. The break in the trend of the volcanic front in the northern extremity of the volcanic zone is thought to reflect the complexity of the crustal stress field above a major segment boundary in the subducting plate. The fields of mafic monogenetic centers also occur in this region. In comparison with the southern part of the CVZ, the general paucity of older volcanic edifices north of 17° S suggests a more recent onset of volcanism north of this latitude probably resulting from the oblique subduction of the Nazca ridge and the consequent northward migration of its intersection with the Pere-Chile trench. This, coupled with the lack of any large silicic caldera systems and youthful dacite domes, like those found further south, suggest that there are real differences between the volcanic evolution of different parts of the CVZ. 相似文献
14.
Eleanor Donoghue Valentin R. Troll Chris Harris Aoife O'Halloran Thomas R. Walter Francisco J. Pérez Torrado 《Journal of Volcanology and Geothermal Research》2008
The Miocene Tejeda caldera on Gran Canaria erupted ~ 20 rhyolite–trachyte ignimbrites (Mogán Group 14–13.3 Ma), followed by ~ 20 phonolitic lava flows and ignimbrites (Fataga Group 13–8.5 Ma). Upper-Mogán tuffs have been severely altered immediately within the caldera margin, whereas extra-caldera Mogán ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid–rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar, plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid–rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The δ18O values of the altered intra-caldera tuffs are significantly higher than in unaltered extra-caldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extra-caldera ignimbrites have δD values between − 110‰ and − 173‰, which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The δD values of the altered intra-caldera tuffs range from − 52‰ to − 131‰, with ambient meteoric water at the alteration site estimated at ca. − 15‰. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock δD values of around − 45‰, given that ?Dclay–water is ca. − 30‰ at 100 °C, and decreases in magnitude at higher temperatures. All altered tuff samples have δD values that are substantially lower than − 45‰, indicating interaction with a meteoric water source with a δD value more negative than − 15‰, which may have been produced in low-temperature steam fumaroles. Supported by numerical modeling, our Gran Canaria data reflect the near-surface, epithermal part of a larger, fault-controlled hydrothermal system associated with the emplacement of the high-level Fataga magma chamber system. In this near-surface environment, fluid temperatures probably did not exceed 200–250 °C. 相似文献
15.
Gerardo J. Aguirre-Díaz Margarita López-Martínez 《Journal of Volcanology and Geothermal Research》2009,179(1-2):133-148
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
16.
Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States 总被引:2,自引:0,他引:2
Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United
States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin
and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites
are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are
interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the
mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation
was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg
ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and
polybaric crystal fractionation were all important processes in generating this Oligocene suite. In contrast, most of the
rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830–1,050°C), Sr-and Al-poor,
and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely
imposed by their derivation from parental basalt (with low fH2O and low fO2) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation
of plagioclase and retarded crystallization of Fe–Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment.
Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation,
and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite
were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte
trend.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
17.
High-magnesium andesites associated with basalts erupted after the opening of the Sea of Japan are present at Saga–Futagoyama in northwest Kyushu, southwest Japan. High Mg/(Mg + Fe) [=0.84] of orthopyroxene phenocrysts and bulk rock Mg–Fe–Ni compositions suggest that these high-magnesium andesites were originally primitive melts insignificantly modified in crustal magma chambers. KDCa–Na [= (Ca/Na)pl/(Ca/Na)bulk rock] ranges from 1.21 to 0.97 and suggests that the high-magnesium andesite magmas would originally have contained H2O less than 1.8 wt.%. Nb/La does not show a negative correlation with respect to SiO2. These lines of evidence indicate that hydrous components derived from the subducting slab would not have played a significant role in the genesis of the high-magnesium andesite magmas. Instead, the normative olivine − quartz − [CaTs + Jd] compositions and a negative correlation between Sr/Nd and SiO2 indicate that the basalt-high-magnesium andesite association would have been formed by multi-stage partial melting of relatively anhydrous source at pressure ranging from 1.5 to 0.5 GPa. 相似文献
18.
Tanna island is part of a large volcanic complex mainly subsided below sea-level. On-land, two series of hydroclastic deposits
and ignimbrites overlie the subaerial remains of a basal, mainly effusive volcano. The ‘Older’ Tanna Ignimbrite series (OTI),
Late Pliocene or Pleistocene in age, consists of ash flows and ash- and scoria-flow deposits associated with fallout tephra
layers, overlain by indurated pumice-flow deposits. Phreatomagmatic features are a constant characteristic of these tuffs.
The ‘younger’ Late Pleistocene pyroclastics, the Siwi sequence, show basal phreatomagmatic deposits overlain by two successive
flow units, each comprising a densely welded layer and a nonwelded ash-flow deposit. Whole-rock analyses of 17 juvenile clasts
from the two sequences (vitric blocks from the phreatomagmatic deposits, welded blocks, scoriaceous bombs and pumices from
the ignimbrites) show basaltic andesite and andesite compositions (SiO2=53–60%). In addition, 296 microprobe analyses of glasses in these clasts show a wide compositional range from 51 to 69% SiO2. Dominant compositions at ∼54, 56, 58.5 and 61–62% SiO2 characterize the glass from the OTI. Glass compositions in the lower – phreatomagmatic – deposits from the Siwi sequence
also show multimodal distribution, with peaks at SiO2=55, 57.5, 61–62 and 64% whereas the upper ignimbrite has a predominant composition at 61–62% SiO2. In both cases, mineralogical data and crystal fractionation models suggest that these compositions represent the magmatic
signature of a voluminous layered chamber, the compositional gradient of which is the result of fractional crystallization.
During two major eruptive stages, probably related to two caldera collapses, the OTI and Siwi ignimbrites represent large
outpourings from these magmatic reservoirs. The successive eruptive dynamics, from phreatomagmatic to Plinian, emphasize the
role of water in initiating the eruptions, without which the mafic and intermediate magmas probably would not have erupted.
Received: February 19, 1993/Accepted October 10, 1993 相似文献
19.
The 29.5 Ma Wah Wah Springs Formation which erupted from the Indian Peak Caldera has an estimated volume of > 3900 km3 making it one of the largest ignimbrites on earth. The magma was calc-alkaline, dacitic (68 wt. % SiO2) and phenocryst-rich (38 vol.%). Phenocrysts include plagioclase (An 47), magnesio-hornblende, Mg-biotite, quartz, Fe-Ti oxides, diopsidic-augite, and rare Ca-poor pyroxene, in order of decreasing abundance. Apatite, zircon and pyrrhotite occurs as inclusions within phenocrysts. Atmospheric glass losses (1040 km3) account for bulk-rock compositions that have SiO2 contents ranging from 63 to 67 wt.%. Glass compositions are high-silica rhyolite.Phenocrysts equilibrated at temperatures ranging from about 790 to 850°C and oxygen fugacities approximately 2.6 log units above the QFM buffer. Confining pressure estimates using the aluminum-in-hornblende geobarometer calibrated for calc-alkaline volcanic rocks suggest a mean pressure of 230±50 MPa corresponding to 7.5±1.5 km depth. These estimates are consistent with caldera formation accompanying emplacement.Crystal compositions for phenocrysts and mineral inclusions within phenocrysts are remarkably homogeneous throughout the outflow tuff, although minor zoning does occur. Given the dacitic composition of the magma, the weakly zoned phenocryst population cannot be modeled to produce the observed high-silica glass (melt) indicating open-system behavior for the magma. The high-silica rhyolite glass is interpreted to be an artifact of efficient magma mixing accompanying addition of highly evolved magma, or melt to intermediate composition magma. Mixing was followed by magma hybridization. Additional support for this hybridization model includes: (1) physically and chemically distinct populations of augite; (2) minor but unbiquitous resorbed plagioclase, biotite and hornblende phenocrysts; and (3) reverse zoning in some of the plagioclase euhedra within pumice lapilli. 相似文献
20.
True rhyolites are found in two contrasting occurrences in Central America: in recent obsidian domes in the basalt-rhyolite association of southeastern Guatemala, and in late Tertiary ignimbrites in Honduras and adjacent parts of Nicaragua. Both are on the inner side of the main volcanic axis in a region that is visibly underlain by older metamorphic and plutonic rocks. They have not been found in southern Central America where the basement series consists only of older volcanic rocks, eugeosynclinal sediments, and peridotite. Rhyolite obsidian has erupted alternately with basalts in the Obrajuelo Complex of southeastern Guatemala. Cumulate rocks and moderate compositional variations in the basalts provide evidence of limited crystal fractionation, but abundant venocrysts and partially fused xenoliths indicate that partial melting of the plutonic and metamorphic basement may also have been important. Extremely siliceous rhyolite ignimbrites with a volume of several thousands of cubic kilometers closely resemble the rhyolite obsidians in their major element compositions, but in contrast to the totally glassy obsidians they contain abundant phenocrysts of quartz and alkali feldspar. A few ignimbrites contain inclusions of partially melted basement rocks. The basalts with which they are associated have a limited compositional range. Melting experiments performed on rocks of the basement series, together with chemical, isotopic, and volumetric relations, indicate that the ignimbrites are products of partial fusion of the basement series, while obsidians of the Obrajuelo Complex have differentiated from more basic magmas by a process other than crystal fractionation. 相似文献