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21.
Apatite is a common accessory mineral in intermediate and felsic igneous rocks. Because apatite has a wide range of trace-element compositions as well as strong resistance to diagenetic alteration, the trace-element composition of apatite has been used for tracing petrogenetic processes of plutonic bodies as well as the tephrochronology of Paleozoic tephras. However, it has been little used in Quaternary volcaniclastic deposits. Our studies of apatite trace-element compositions in representative Quaternary ignimbrites and their corresponding co-ignimbrite ashes in Japan demonstrated that such compositions are not affected by welding and can be successfully used for discrimination of, and correlation between, tephras and ignimbrites. We also could distinguish different tephra deposits and ignimbrites from the same caldera. Although most apatite trace-element compositions did not change throughout the succession of ejecta from a single eruption, the Fe contents in apatite varied across stratigraphic horizons. Consequently, we were able to correlate the Fe contents of apatite phenocrysts in specific ignimbrite horizons and their corresponding co-ignimbrite ashes and thus identify the phase of the eruption during which the major part of a co-ignimbrite ash was released. 相似文献
22.
Jonathan Obrist-Farner Andreas Eckert Marek Locmelis James L. Crowley Byron Mota-Vidaure Emanuele Lodolo Joshua Rosenfeld Edward Duarte 《Basin Research》2020,32(6):1347-1364
The Lake Izabal Basin in Guatemala is a major pull-apart basin along the sinistral Polochic Fault, which is part of the North American and Caribbean plate boundary. The basin infill contains information about the tectonic and sedimentological processes that have imparted a significant control on its sedimentary section. The inception of the basin has been linked to the relative importance of the Polochic Fault in the tectonic history of the plate boundary; yet, its sedimentological record and its inception age have been poorly documented. This study integrates diverse datasets, including industry reports, well logs and reports, well cuttings, vintage seismic data, outcrop observations and geochronological data to constrain the initial infill and age of inception of the basin. The integrated data show that during the Oligocene–Miocene, a marine carbonate platform was established in the region which was later uplifted and eroded in the early Miocene. The fluvial–lacustrine deposits above this carbonate platform are part of the initial infill of the basin and are constrained with zircon weighted-mean 206Pb/238U ages of 12.060 ± 0.008 from a volcanic tuff ~30 m above the unconformity. Sandstone, mudstone and coal dominate the interval from 12 to 4 Ma, with an increase in conglomerate correlating to the uplift of the Mico Mountains and San Gil Hill at 4 Ma. Fault switch activity between the Polochic and Motagua faults has been hypothesized to explain total offset along the Polochic Fault and the geologic and geodetic slip rates along the two faults. The 12 Ma age determined for the initial infill of the basin confirms this hypothesis. Consequently, our study confirms that at ~12 Ma the Polochic Fault served as the main fault of the plate boundary with inferred slip rates ranging from 13 to 21 mm/yr with a strong possibility that the Polochic Fault was, at some point between 15 Ma and 7 Ma, the only active fault of the plate boundary. The results of this study show that tectonic records preserved in sediments of strike-slip basins improve the understanding of the relative significance of individual faults and the implications with respect to strain partitioning throughout its tectonic history. 相似文献
23.
24.
Formation of high-grade ignimbrites Part II. A pyroclastic suspension current model with implications also for low-grade ignimbrites 总被引：2，自引：2，他引：0
Armin Freundt 《Bulletin of Volcanology》1999,60(7):545-567
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
25.
Deterioration mechanisms of tuffs in Midas monument 总被引：3，自引：0，他引：3
Slightly weathered white and pink tuffs of the Midas monument have deterioration problems. In this study, depths and characteristics of the weathering zones developed within the tuffs are investigated through optical microscopy, X-ray diffractometry (XRD), chemical analyses, scanning electron microscopy (SEM) and some index parameters. Accelerated weathering tests including wetting–drying, freezing–thawing and salt crystallization are performed, and durability assessment methods are used to predict the durabilities of the tuffs. The findings are compared with field observations. By examining quantitative weathering indices and comparing them with thin section studies, it is found that thin section analyses of the crystals, LoI, and WPI are good indicators to quantify the depth of weathering for the tuffs. However, thin section studies have limited value for fine-grained tuffaceous matrix. The chemical weathering of the tuffs produces weathered zones that are 4.5-cm thick within the white tuff and 2.5-cm thick within the pink tuff. Physical weathering causes scaling of outer layers of the tuffs and fracturing of feldspars along their cleavage planes. However, variations of the index properties of the tuffs due to weathering are not so significant to quantify the weathering depths in the tuffs. Among the accelerated weathering tests, salt crystallization is found to be the most destructive environmental condition. Pigeon droppings rich in NO31− are found to be the main source of soluble salt at the Midas monument. The salt transported up by capillary rise due to surface water causes spalling of the tuffs in the capillary zone. Surface water and salt of any kind in the close vicinity of the monument should be totally eliminated for the purpose of conservation. Field observations and the durability equations reveal that the white tuff is less durable than the pink tuff. Wet-to-dry strength ratio yields a better stone durability assessment among various durability methods used in this study. 相似文献
26.
Ranking welding intensity in pyroclastic deposits 总被引：1，自引：1，他引：1
Welding of pyroclastic deposits involves flattening of glassy pyroclasts under a compactional load at temperatures above the glass transition temperature. Progressive welding is recorded by changes in the petrographic (e.g., fabric) and physical (e.g., density) properties of the deposits. Mapping the intensity of welding can be integral to studies of pyroclastic deposits, but making systematic comparisons between deposits can be problematical. Here we develop a scheme for ranking welding intensity in pyroclastic deposits on the basis of petrographic textural observations (e.g., oblateness of pumice lapilli and micro-fabric orientation) and measurements of physical properties, including density, porosity, point load strength and uniaxial compressive strength. Our dataset comprises measurements on 100 samples collected from a single cooling unit of the Bandelier Tuff and parallel measurements on 8 samples of more densely welded deposits. The proposed classification comprises six ranks of welding intensity ranging from unconsolidated (Rank I) to obsidian-like vitrophyre (Rank VI) and should allow for reproducible mapping of subtle variations in welding intensity between different deposits. The application of the ranking scheme is demonstrated by using published physical property data on welded pyroclastic deposits to map the total accumulated strain and to reconstruct their pre-welding thicknesses.Editorial Responsibility: T. Druitt 相似文献
27.
若干地区的研究证明,熔结凝灰岩是液体熔离——泡沫熔岩流生成的。无色玻屑及其间隙中的褐色玻璃质胶结物为物质的熔体属性提供了证据。熔浆中丰富的水主要来自火山口湖水的向下渗漏,部分属岩浆水。未脱玻化的岩石中无色玻屑和褐色玻璃质胶结物成分分析以及褐色玻璃为主要组分的火焰石的结构和成分分析表明,熔浆中发生过熔离作用。贫水较富铁的液体球滴因密度大发生了重力分异,于负荷压力及粘性流动条件下凝结为火焰石。成岩后岩石发生了广泛的重结晶和自变质作用。德兴地区发现大量的排气筒为一奇景。 相似文献
28.
Collapse mechanism of the Paleogene Sakurae cauldron, SW Japan 总被引：1，自引：0，他引：1
The Paleogene Sakurae cauldron of SW Japan is characterized by a nested structure with a polygonal outline (21×13 km2) including a circular collapsed part (5 km in diameter). Total thickness of the caldera infill amounts to 2,000 m. The lower member of the infill consists mainly of felsic crystal tuff and lesser intercalated andesitic lava flows, whereas the upper member is composed of high-grade ignimbrite capped with a large rhyolitic lava dome. These members represent the first and second stage eruptions, respectively. Faults bounding the cauldron rim comprise intersecting radial and concentric faults, producing the polygonal outline of this cauldron. The primary collapse of this cauldron thus occurred as a polygonal caldera basin where products of the first stage eruption accumulated. In contrast, the inner collapse part is defined by a ring fracture system. This sector subsided concurrently with accumulation of the high-grade ignimbrite of the second stage eruption. This inner circular collapse thus represents syn-eruptional subsidence concurrent with the climactic eruption. Magma drainage during the first stage probably induced outward-dipping ring fractures in the chamber roof. Opening of the ring fractures following subsidence of the central bell-jar block caused rapid evacuation of magma as voluminous pumice flows, even though magma pressure may have decreased to some degree. 相似文献
29.
塔里木溢流玄武岩省的巨型长英质热火山碎屑流爆发记录 总被引：1，自引：1，他引：0
在塔里木盆地西北缘的柯坪地区二叠系库普库兹满组和开派兹雷克组玄武岩之间发现厚层长英质火山碎屑岩层序。该层序包括可见含交错层理的空落火山灰层、三层含增生火山砾的火山灰、熔结凝灰岩和再沉积熔结凝灰岩。层序下部为与其准同时喷发的玄武质火山碎屑岩和玄武质熔岩流。利用锆石U-Pb法确定熔结凝灰岩层的喷发年龄为290.9±1.3Ma(MSWD=1.12),该年龄限定了库普库兹满组玄武岩喷发的截止时间。长英质火山碎屑岩层序中的增生火山砾由粒度250μm的长英质玻屑组成,且长宽比均1.5。根据形貌、结构和岩相学特征,将增生火山砾分为三类,分别对应热火山碎屑流从起始(TypeⅡ,coated ash pellet)到极盛(TypeⅠ,accretionary lapillus)再逐渐衰弱(TypeⅢ,ash pellet)的过程。由于喷发规模巨大,该火山层序很可能广泛分布于盆地内,可能是确定全盆地溢流玄武岩喷发时限的一个关键标志层。 相似文献
30.
Complex proximal deposition during the Plinian eruptions of 1912 at Novarupta,Alaska 总被引：2，自引：1，他引：1
Bruce?F.?HoughtonEmail author C.?J.?N.?Wilson J.?Fierstein W.?Hildreth 《Bulletin of Volcanology》2004,66(2):95-133
Proximal (<3 km) deposits from episodes II and III of the 60-h-long Novarupta 1912 eruption exhibit a very complex stratigraphy, the result of at least four transport regimes and diverse depositional mechanisms. They contrast with the relatively simple stratigraphy (and inferred emplacement mechanisms) for the previously documented, better known, medial–distal fall deposits and the Valley of Ten Thousand Smokes ignimbrite. The proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta, and numerous thin complex deposits of pyroclastic density currents (PDCs) with no regional analogs. The locally dispersed component of the fall deposits forms sector-confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100–300 m (cf. several kilometers for the medial–distal fall deposits). This locally dispersed fall material (and many of the associated PDC deposits) is rich in andesitic and banded pumices and richer in shallow-derived wall-rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition. There are no marked contrasts in grain size in the near-vent deposits, however, between locally and widely dispersed beds, and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial–distal samples. Associated PDC deposits form a spectrum of facies from fines-poor, avalanched beds through thin-bedded, landscape-mantling beds to channelized lobes of pumice-block-rich ignimbrite. The origins of the Novarupta near-vent deposits are considered within a spectrum of four transport regimes: (1) sustained buoyant plume, (2) fountaining with co-current flow, (3) fountaining with counter-current flow, and (4) direct lateral ejection. The Novarupta deposits suggest a model where buoyant, stable, regime-1 plumes characterized most of episodes II and III, but were accompanied by transient and variable partitioning of clasts into the other three regimes. Only one short period of vent blockage and cessation of the Plinian plume occurred, separating episodes II and III, which was followed by a single PDC interpreted as an overpressured "blast" involving direct lateral ejection. In contrast, regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume, which were being strongly affected by short-lived instabilities. These instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent. Of the parameters that control explosive eruptive behavior, only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products.Editorial responsibility: R. Cioni 相似文献