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Volcn Ollage (2117'S) is a large stratovolcano that liesslightly east of the main axis of Quaternary Volcanoes in theAndean Central Volcanic Zone (CVZ). Euptive products range frombasaltic andesite to dacite and define a high-K, calc-alkalinesuite. This compositional range is similar to the collectivecompositional range of the other stratovolcanoes in the CVZ,and it provides a record of both early and late-stage differentiationprocesses operating at the stratovolcanoes. The volumetrically dominant andesitic and dacitic lavas aredivided into four eruptive series on the basis of vent locationsand petrography. In ascending stratigraphic order they are:the Vinta Loma, Chasca Orkho, post-collapse, and La Celosa series.Whole-rock compositions of the lavas are remarkably similarregardless of eruptive series. Variations in phenocryst assemblagesand magmatic fo2 however, suggest differences in subliquidusvolatile contents for magma chambers developed beneath the summitof the volcano versus those developed beneath the flanks. Basalticandesite magmas are principally preserved as quenched inclusionswithin the andesitic and dacitie lava flows. Large ranges inisotopic ratios over a narrow compositional range indicate thatthe basaltic andesites were derived by crystal fractionationcoupled with large amounts of crustal assimilation. IncreasingCe/Yb ratios with decreasing Yb contents further suggest thatthis initial stage of differentiation occurred at deep crustallevels where garnet was stable. Additional supporting evidencefor differentiation in the deep crust includes isotopic andtrace element compositions that indicate assimilation by thebasaltic andesite magmas of a crust different from upper-crustalrocks exposed at present in the region. Whole-rock major and trace element trends of the dacitic lavascan be simulated largely by fractional crystallization of parentalandesitic magma. The fractionating assemblages for the differenteruptive series are consistent with the observed modes of theparent magmas. Small increases in Sr isotope ratios with increasingRb contents indicate that the fractionating magmas also assimilatedsmall amounts of wall rocks similar in composition to the upper-crustalbasement to the volcano. Consideration of the chemical trends, mineral compositions,and eruptive history of Ollage rocks permits construction ofa model for the evolution of shallow crustal magma chambersbeneath the stratovolcanoes in the CVZ. At a relatively maturestage, the magma chambers may be compositionally, thermally,and density stratified. Temperatures estimated from Fe-Ti oxideand pyroxene thermometry for the chambers beneath Ollage rangefrom 1000 to 790C with increasing SiO2 from 59 to 67 wt.% inthe upper reaches, and from 1150 to 1020C with increasing SiO2from 53 to 59 wt.% in the lower reaches. The occurrence of basalticandesite magmatic inclusions within the intermediate lavas andthe repeated eruption of monotonous composition andesitic magmasindicate that the shallow chambers are periodically replenishedwith parental basaltic andesite magmas. Ubiquitous, reversely zoned plagioclase and pyroxene phenocrystsin the lavas at Ollage suggest that convective cooling of thebasaltic andesite releases buoyant derivative liquid that mixeswith the overlying intermediate-composition body of the chambers.Further crystallization and differentiation of the intermediatemagmas may take place in solidification zones at the boundariesof the magma chambers. If so, the return of residual liquidfrom the crystallizing margins and mixing with the interiorare highly efficient such that magma differentiation can bemodeled as a simple, homogeneous, fractional crystallizationprocess. 相似文献
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Dry deposition of particles 总被引：5，自引：0，他引：5
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DAVIDSON BLACK 《地质学报》1926,(Z1)
A rich fossiliferous deposit at Chou Kou Tien 70 li to the south west of Peking was first discovered in the summer of 1921 by Dr. J. G. Andersson and later surveyed and partially excavated by Dr. O Zdansky. A preliminary roport on the site was published by Dr. Andersson in March 1923 (Mem. Geol. 相似文献
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The island of St Martin lies inthe inactive part of the northernLesser Antilles island arc. The island consists of volcaniclasticsediments overlain and intruded by volcanic and plutonic rocks,which are in turn overlain by Miocene limestones. The extrusiveand intrusive rock suites are closely spaced in time (around27 Ma) although field relations suggest that the volcanic rockswere intruded by the plutons. Pluton emplacement gave rise tothermal metamorphism of the volcanic and volcaniclastic carapace,and to widespread hydrothermal alteration throughout the island. Geochemically, the igneous rocks of St Martin form a mildlytholeiitic to calc-alkaline typical subduction-related suite.The extrusive rocks are basalts to andesites, and the magmasappear to have differentiated largely through fractional crystallizationof plagioclase, clinopyroxene, and olivine. The REE displayflat chondrite-normalized patterns, with no significant Eu anomalydespite convincing evidence for plagioclase fractionation. Theplutonic rocks are more silica-rich diorites to granites, containingplagioclase, amphibole, and, less commonly, K-feldspar, sphene,zircon, and pyroxene. REE patterns are slightly LREE enrichedbut display distinct negative Eu anomalies. The fractionationof amphibole and accessory phases may have been important inthe evolution of the plutonic suite, as REE contents do notincrease overall with differentiation. Sr and Nd isotopic ratios of the St Martin suite form restrictedranges which vary little with differentiation, or between theextrusive and intrusive suiteSi 87Sr/86Sr ratios are slightlyhigher and 143Nd/l44Nd slightly lower than for volcanic rocksuites from the currently active northern Lesser Antilles volcanicarc. Some of the high 87Sr/86Sr ratios are explained in termsof hydrothermal alteration involving a high 87Sr/86Sr fluid,associated with pluton emplacement. Pb isotope ratios are similarto those of the currently active northern Lesser Antilles arc,and correlate with SiO2. Such correlations, together with largeranges of incompatible (and immobile) trace element ratios suggestthat open-system differentiation occurred during the evolutionof the St Martin suite. The composition of magma sources in the northern Lesser Antillesarc apparently has not changed significantly over the last 30Ma, despite a westward shift in the locus of arc magmatism.Addition of a slab-derived fluid to the mantle wedge is responsiblefor the high relative abundances of large ion lithophile elements(LILE) and enrichment in radiogenic Pb and Sr relative to mid-oceanridge basalt (MORB). Subsequent differentiation may involveassimilation of the arc basement in St Martin, which is believedto consist of Cretaceous to early Tertiary arc material, similarto that encountered in the Greater Antilles.
*Present address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721 相似文献
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Jemma DAVIDSON Henner BUSEMANN Ian A. FRANCHI 《Meteoritics & planetary science》2012,47(11):1748-1771
Abstract– Interplanetary dust particles (IDPs) are the most primitive extraterrestrial material available for laboratory studies and may, being likely of cometary origin, sample or represent the unaltered starting material of the solar system. Here we compare IDPs from a “targeted” collection, acquired when the Earth passed through the dust stream of comet 26P/Grigg‐Skjellerup (GSC), with IDPs from nontargeted collections (i.e., of nonspecific origin). We examine both sets to further our understanding of abundances and character of their isotopically anomalous phases to constrain the nature of their parent bodies. We identified ten presolar silicates, two oxides, one SiC, and three isotopically anomalous C‐rich grains. One of seven non‐GSC IDPs contains a wealth of unaltered nebula material, including two presolar silicates, one oxide, and one SiC, as well as numerous δD and δ15N hotspots, demonstrating its very pristine character and suggesting a cometary origin. One of these presolar silicates is the most 17O‐rich discovered in an IDP and has been identified as a possible GEMS (glass with embedded metal and sulfides). Organic matter in an anhydrous GSC IDP is extremely disordered and, based on Raman spectral analyses, appears to be the most primitive IDP analyzed in this study, albeit only one presolar silicate was identified. No defining difference was seen between the GSC and non‐GSC IDPs studied here. However, the GSC collectors are expected to contain IDPs of nonspecific origin. One measure alone, such as presolar grain abundances, isotopic anomalies, or Raman spectroscopy cannot distinguish targeted cometary from unspecified IDPs, and therefore combined studies are required. Whilst targeted IDP populations as a whole may not show distinguishable parameters from unspecified populations (due to statistics, heterogeneity, sampling bias, mixing from other cometary sources), particular IDPs in a targeted collection may well indicate special properties and a fresh origin from a known source. 相似文献
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Diagnostics are presented from an ensemble of high-resolution forecasts that differed markedly in their predictions of the rapid intensification(RI)of Typhoon Rammasun.We show that the basic difference stems from subtle differences in initializations of(a)500-850-h Pa environmental winds,and(b)midlevel moisture and ventilation.We then describe how these differences impact on the evolving convective organization,storm structure,and the timing of RI.As expected,ascent,diabatic heating and the secondary circulation near the inner-core are much stronger in the member that best forecasts the RI.The evolution of vortex cloudiness from this member is similar to the actual imagery,with the development of an inner cloud band wrapping inwards to form the eyewall.We present evidence that this structure,and hence the enhanced diabatic heating,is related to the tilt and associated dynamics of the developing inner-core in shear.For the most accurate ensemble member:(a)inhibition of ascent and a reduction in convection over the up-shear sector allow moistening of the boundary-layer air,which is transported to the down-shear sector to feed a developing convective asymmetry;(b)with minimal ventilation,undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud;and(c)this process seems related to a critical down-shear tilt of the vortex from midlevels,and the vertical phase-locking of the circulation over up-shear quadrants.For the member that forecasts a much-delayed RI,these processes are inhibited by stronger vertical wind shear,initially resulting in poor vertical coherence of the circulation,lesser moisture and larger ventilation.Our analysis suggests that ensemble prediction is needed to account for the sensitivity of forecasts to a relatively narrow range of environmental wind shear,moisture and vortex inner-structure. 相似文献
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ABSTRACT. In the public‐space discourse Los Angeles is usually portrayed as more “anti‐city” than city. Its landscape is overrun by houses, “private‐public” squares and plazas, theme parks, shopping malls, and so on and lacks inclusive public places. Yet this discourse has essentially disdained to contemplate a major public space that contradicts its general thesis: the Los Angeles coast. The coast is meaningful public place in two specific senses. First, it symbolizes Los Angeles as a whole and therefore provides a basis for regional public identity. Second, Angelinos themselves take the coast seriously as a public place, and they have striven to make it inclusive in practice. 相似文献
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Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas 总被引：4，自引：0，他引：4
PERINI GIULIA; FRANCALANCI LORELLA; DAVIDSON JON P.; CONTICELLI SANDRO 《Journal of Petrology》2004,45(1):139-182
Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (87Sr/86Srinitial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy 相似文献