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1.
Kaguyak Caldera lies in a remote corner of Katmai National Park, 375 km SW of Anchorage, Alaska. The 2.5-by-3-km caldera collapsed ~ 5.8 ± 0.2 ka (14C age) during emplacement of a radial apron of poorly pumiceous crystal-rich dacitic pyroclastic flows (61–67% SiO2). Proximal pumice-fall deposits are thin and sparsely preserved, but an oxidized coignimbrite ash is found as far as the Valley of Ten Thousand Smokes, 80 km southwest. Postcaldera events include filling the 150-m-deep caldera lake, emplacement of two intracaldera domes (61.5–64.5% SiO2), and phreatic ejection of lakefloor sediments onto the caldera rim. CO2 and H2S bubble up through the lake, weakly but widely. Geochemical analyses (n = 148), including pre-and post-caldera lavas (53–74% SiO2), define one of the lowest-K arc suites in Alaska. The precaldera edifice was not a stratocone but was, instead, nine contiguous but discrete clusters of lava domes, themselves stacks of rhyolite to basalt exogenous lobes and flows. Four extracaldera clusters are mid-to-late Pleistocene, but the other five are younger than 60 ka, were truncated by the collapse, and now make up the steep inner walls. The climactic ignimbrite was preceded by ~ 200 years by radial emplacement of a 100-m-thick sheet of block-rich glassy lava breccia (62–65.5% SiO2). Filling the notches between the truncated dome clusters, the breccia now makes up three segments of the steep caldera wall, which beheads gullies incised into the breccia deposit prior to caldera formation. They were probably shed by a large lava dome extruding where the lake is today.  相似文献   
2.
Compositional Zoning of the Bishop Tuff   总被引:14,自引:0,他引:14  
Compositional data for >400 pumice clasts, organized accordingto eruptive sequence, crystal content, and texture, providenew perspectives on eruption and pre-eruptive evolution of the>600 km3 of zoned rhyolitic magma ejected as the Bishop Tuffduring formation of Long Valley caldera. Proportions and compositionsof different pumice types are given for each ignimbrite packageand for the intercalated plinian pumice-fall layers that eruptedsynchronously. Although withdrawal of the zoned magma was lesssystematic than previously realized, the overall sequence displaystrends toward greater proportions of less evolved pumice, morecrystals (0·5–24 wt %), and higher FeTi-oxide temperatures(714–818°C). No significant hiatus took place duringthe 6 day eruption of the Bishop Tuff, nearly all of which issuedfrom an integrated, zoned, unitary reservoir. Shortly beforeeruption, however, the zoned melt-dominant portion of the chamberwas invaded by batches of disparate lower-silica rhyolite magma,poorer in crystals than most of the resident magma but slightlyhotter and richer in Ba, Sr, and Ti. Interaction with residentmagma at the deepest levels tapped promoted growth of Ti-richrims on quartz, Ba-rich rims on sanidine, and entrapment ofnear-rim melt inclusions relatively enriched in Ba and CO2.Varied amounts of mingling, even in higher parts of the chamber,led to the dark gray and swirly crystal-poor pumices sparselypresent in all ash-flow packages. As shown by FeTi-oxide geothermometry,the zoned rhyolitic chamber was hottest where crystal-richest,rendering any model of solidification fronts at the walls orroof unlikely. The main compositional gradient (75–195ppm Rb; 0·8–2·2 ppm Ta; 71–154 ppmZr; 0·40–1·73% FeO*) existed in the melt,prior to crystallization of the phenocryst suite observed, whichincluded zircon as much as 100 kyr older than the eruption.The compositions of crystals, though themselves largely unzoned,generally reflect magma temperature and the bulk compositionalgradient, implying both that few crystals settled or were transportedfar and that the observed crystals contributed little to establishingthat gradient. Upward increases in aqueous gas and dissolvedwater, combined with the adiabatic gradient (for the 5 km depthrange tapped) and the roofward decline in liquidus temperatureof the zoned melt, prevented significant crystallization againstthe roof, consistent with dominance of crystal-poor magma earlyin the eruption and lack of any roof-rind fragments among theBishop ejecta, before or after onset of caldera collapse. Amodel of secular incremental zoning is advanced wherein numerousbatches of crystal-poor melt were released from a mush zone(many kilometers thick) that floored the accumulating rhyoliticmelt-rich body. Each batch rose to its own appropriate levelin the melt-buoyancy gradient, which was self-sustaining againstwholesale convective re-homogenization, while the thick mushzone below buffered it against disruption by the deeper (non-rhyolitic)recharge that augmented the mush zone and thermally sustainedthe whole magma chamber. Crystal–melt fractionation wasthe dominant zoning process, but it took place not principallyin the shallow melt-rich body but mostly in the pluton-scalemush zone before and during batchwise melt extraction. KEY WORDS: Bishop Tuff; ignimbrite; magma zonation; mush model; rhyolite  相似文献   
3.
Isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the Mt. St. Helens volcano suggest three different episodes; (1) 40,000–2500 years ago: eruptions of dacite with εNd = +5, εSr = ?10, variable δ18O,206Pb/204Pb ~ 18.76, Ca/Sr ~ 60, Rb/Ba ~ 0.1, La/Yb ~ 18, (2) 2500-1000 years ago: eruptions of basalt, andesite and dacite with εNd = +4 to +8, εSr = ?7 to ?22, variable δ18O (thought to represent melting of differing mantle-crust reservoirs), 206Pb/204Pb= 18.81?18.87, variable Ca/Sr, Rb/Ba, La/Yb and high Zr, (3) 1000 years ago to present day: eruptions of andesite and dacite with εNd = +6, εSr = ?13, δ18O~6‰, variable206Pb/204Pb, Ca/Sr ~ 77, Rb/Ba= 0.1, La/Yb ~ 11. None of the products exhibit Eu anomalies and all are LREE enriched. There is a strong correlation between87Sr/86Sr and differentiation indices. These data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet, but not feldspar, followed by intrusion of this crustal reservoir by mantle-derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components. Since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re-equilibration of Pb isotopic compositions than other components suggesting that Pb is being transported via a fluid phase. The Nd and Sr isotopic compositions lie along the mantle array and suggest that the mantle underneath Mt. St. Helens is not as depleted as MORB sources. There is no indication of seawater involvement in the source region.  相似文献   
4.
The voluminous Pleistocene—Recent Taupo rhyolites typically contain phenocrysts of plagioclase (oligoclase-labradorite), quartz, titanomagnetite, ilmenite, and ferromagnesian silicates. Ferromagnesian assemblages correlate with well defined Fe-Ti oxide equilibration temperature ranges and allow the rhyolites to be subdivided as follows: (1) Cummingtonite (c)—calcic hornblende (hb)—orthopyroxene (opx); 725–755°C, (2) Hb-opx, 750–825°C, (3) Biotite-hb-(c-opx), 720–765°C, (4) Opx-clinopyroxene (cpx), 860–915°C, (5) Fe olivine-opx-cpx, one sample with temperature of 900°C. Plagioclase and orthopyroxene phenocryst compositions typically exhibit a range of composition up to ~20 mol.%. Calculated average phenocryst equilibration pressures (P total) range between 0.5–4.9 kb, and average 2.2 kb (~7–8 km depth), indicating upper crustal crystallization. These calculations are very sensitive to variations in phenocryst composition. Calculated \(/_{{\text{H}}_2 {\text{O}}} \) for the amphibole and biotite-bearing rhyolites indicate phenocryst equilibration under \(P_{{\text{H}}_2 {\text{O}}} \simeq P_{{\text{total}}} \) , with \(X_{{\text{H}}_2 {\text{O}}} \) ~0.17–0.24 (5–8 wt. %). The precipitation of cummingtonite is thus temperature dependent, the upper limit being close to 760°C. Eruptive mechanisms of the lavas, pumices, and ash-flow deposits are evidently not primarily controlled by temperature, P total, \(P_{{\text{H}}_2 {\text{O}}} \) , or crystal content of the magmas, and explanations must lie in kinetic and fluid dynamic behavior of the magmas. For the Taupo rhyolites, it is suggested that the critical size of a magma body (i.e. Rayleigh number) is a controlling factor in that it will influence the convective regime; fully turbulent convection is deduced to have occurred within the larger magma bodies. One consequence is intense vesiculation, prior to eruption, within the uppermost zones of these magma chambers, and the voluminous pumice deposits are believed to emanate from such chambers. Oscillatory compositional zoning within pyroxene phenocrysts is consistent with magma convection.  相似文献   
5.
Many current watershed modeling efforts now incorporate surface water and groundwater for managing water resources since the exchanges between groundwater and surface water need a special focus considering the changing climate. The influence of groundwater dynamics on water and energy balance components is investigated in the Snake River Basin (SRB) by coupling the Variable Infiltration Capacity (VIC) and MODFLOW models (VIC‐MF) for the period of 1986 through 2042. A 4.4% increase in base flows and a 10.3% decrease in peak flows are estimated by VIC‐MF compared to the VIC model in SRB. The VIC‐MF model shows significant improvement in the streamflow simulation (Nash‐Sutcliffe efficiency [NSE] of 0.84) at King Hill, where the VIC model could not capture the effect of spring discharge in the streamflow simulation (NSE of ?0.30); however, the streamflow estimates show an overall decreasing trend. Two climate scenarios representing median and high radiative‐forcings such as representative concentration pathways 4.5 and 8.5 show an average increase in the water table elevations between 2.1 and 2.6 m (6.9 and 8.5 feet) through the year 2042. The spatial patterns of these exchanges show a higher groundwater elevation of 15 m (50 feet) in the downstream area and a lower elevation of up to 3 m (10 feet) in the upstream area. Broadly, this study supports results of previous work demonstrating that integrated assessment of groundwater‐surface water enables stakeholders to balance pumping, recharge and base flow needs and to manage the watersheds that are subjected to human pressures more sustainably.  相似文献   
6.
 The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km3/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km3 debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km2 south of the volcano. A gravitationally metastable 2-to 3-km3 reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y. Received: 24 June 1996 / Accepted: 6 December 1996  相似文献   
7.
Geology of the peralkaline volcano at Pantelleria,Strait of Sicily   总被引:1,自引:1,他引:1  
Situated in a submerged continental rift, Pantelleria is a volcanic island with a subaerial eruptive history longer than 300 Ka. Its eruptive behavior, edifice morphologies, and complex, multiunit geologic history are representative of strongly peralkaline centers. It is dominated by the 6-km-wide Cinque Denti caldera, which formed ca. 45 Ka ago during eruption of the Green Tuff, a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls, surges, and pyroclastic flows. Soon after collapse, trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it. Progressive chemical evolution of the chamber between 45 and 18 Ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera. Beginning about 18 Ka ago, inflation of the chamber opened old ring fractures and new radial fractures, along which recently differentiated pantellerite constructed more than 25 pumice cones and shields. Continued uplift raised the northwest half of the intracaldera trachyte cone 275 m, creating the island's present summit, Montagna Grande, by trapdoor uplift. Pantellerite erupted along the trapdoor faults and their hingeline, forming numerous pumice cones and agglutinate sheets as well as five lava domes. Degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18-3-Ka episode led to entrainment of subjacent, crystal-rich, pantelleritic trachyte magma as crenulate inclusions. Progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0.82 km3 of magma erupted during the episode.The 45-Ka-old caldera is nested within the La Vecchia caldera, which is thought to have formed around 114 Ka ago. This older caldera was filled by three widespread welded units erupted 106, 94, and 79 Ka ago. Reactivation of the ring fracture ca. 67 Ka ago is indicated by venting of a large pantellerite centero and a chain of small shields along the ring fault. For each of the two nested calderas, the onset of postcaldera ring-fracture volcanism coincides with a low stand of sea level.Rates of chemical regeneration within the chamber are rapid, the 3% crystallization/Ka of the post-Green Tuff period being typical. Highly evolved pantellerites are rare, however, because intervals between major eruptions (averaging 13–6 Ka during the last 190 Ka) are short. Benmoreites and mugearites are entirely lacking. Fe-Ti-rich alkalic basalts have erupted peripherally along NW-trending lineaments parallel to the enclosing rift but not within the nested calderas, suggesting that felsic magma persists beneath them. The most recent basaltic eruption (in 1891) took place 4 km northwest of Pantelleria, manifesting the long-term northwestward migration of the volcanic focus. These strongly differentiated basalts reflect low-pressure fractional crystallization of partial melts of garnet peridotite that coalesce in small magma reservoirs replenished only infrequently in this continental rift environment.  相似文献   
8.
The Precambrian trondhjemitic Twilight Gneiss (Twilight Granite of Cross and Howe, 1905b) of the West Needle Mountains, southwestern Colorado, and its interlayered amphibolite and metarhyodacite yield a Rb-Sr isochron of 1,805±35 m.y. A low initial Sr87/Sr86 ratio of 0.7015 implies that metamorphism of these rocks to amphibolite facies took place soon after their emplacement. The mild metamorphism of Uncompahgran age, prior to 1,460 m.y. ago, and Laramide volcanism did not affect the Rb-Sr system in the Twilight. Rb contents of 26.5 to 108 ppm, Sr contents of 114 to 251 ppm, and K2O percentages of 1.23 to 3.64 in the Twilight Gneiss, in conjunction with high K/Rb ratios and the low initial ratio of Sr87/Sr86, lend support to geologic data that suggest the Twilight originated as volcanic or hypabyssal igneous rocks in a basaltic volcanic pile.Publication authorized by the Director, U.S. Geological Survey.  相似文献   
9.
10.
A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (230Th/238U) > 1 and span a large range in (230Th/232Th) ratios, and most basalts have higher (230Th/232Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~ 20–25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate 230Th excesses (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by 187Os/188Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large 230Th excesses (25–48%) and high 187Os/188Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization.  相似文献   
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