The bimodal NW Etendeka province is located at the continentalend of the Tristan plume trace in coastal Namibia. It comprisesa high-Ti (Khumib type) and three low-Ti basalt (Tafelberg,Kuidas and Esmeralda types) suites, with, at stratigraphicallyhigher level, interstratified high-Ti latites (three units)and quartz latites (five units), and one low-Ti quartz latite.Khumib basalts are enriched in high field strength elementsand light rare earth elements relative to low-Ti types and exhibittrace element affinities with Tristan da Cunha lavas. The unradiogenic206Pb/204Pb ratios of Khumib basalts are distinctive, most plottingto the left of the 132 Ma Geochron, together with elevated 207Pb/204Pbratios, and SrNd isotopic compositions plotting in thelower 143Nd/144Nd part of mantle array (EM1-like). The low-Tibasalts have less coherent trace element patterns and variable,radiogenic initial Sr ( 相似文献
The simultaneous eruption in 1996 of andesite from Karymskyvolcano and of basalt from the Academy Nauk vent 6 km away appearsto provide a case of mafic recharge of an andesite reservoirfor which the time of recharge is exactly known and direct samplesof the recharging magma are available. The explosive phreato-magmaticeruption of basalt was terminated in less than 24 h, whereasandesite erupted continuously during the following 4 years.Detailed petrological study of volcanic ash, bombs and lavasof Karymsky andesite erupted during the period 19961999provides evidence for basaltic replenishment at the beginningof the eruptive cycle, as well as a record of compositionalvariations within the Karymsky magma reservoir induced by basalticrecharge. Shortly after the beginning of the eruption the compositionof the matrix glass of the Karymsky tephra became more maficand then, within 2 months, gradually returned to its originalstate and remained almost constant for the following 3 years.Further evidence for basaltic replenishment is provided by thepresence of xenocrysts of basaltic origin in the andesite ofKarymsky. A conspicuous portion of the plagioclase phenocrystsin the Karymsky andesite has calcic cores, with compositionsand textures resembling those of plagioclases in the AcademyNauk basalt. Similarly, the earlier portion of the andesiteof the eruption sequence contains rare olivines, which occuras resorbed cores in pyroxenes. The composition of the olivinematches that of olivines in the Academy Nauk basalt. The sequenceof events appears to be: (1) injection of basaltic magma intothe Karymsky chamber with immediate, compensating expulsionof pre-existing chamber magma from the Karymsky central vent;(2) direct mixing of basaltic and andesitic magmas with dispersalof phenocrysts associated with the basalt throughout the andesiteso that newly mixed magma appeared at the vent within 2 months;(3) re-establishment of thermal and chemical equilibrium withinthe reservoir involving crystallization in the new hybrid liquid,which returned the melt composition to normal,formed rims on inherited calcic plagioclase, and caused theresorption of dispersed olivine xenocrysts. Taken together,these findings indicate that the Karymsky magma reservoir wasrecharged by basalt at the onset of the 1996 eruptive cycle.The rapidity and thoroughness of mixing of the basalt with thepre-existing andesite probably reflects the modest contrastin temperature, viscosity, and density between the two magmas. KEY WORDS: Karymsky; Kamchatka; magma mixing; andesite; volcanic glass; plagioclase相似文献
It has been well studied that the γ-function explicit method can be effective in providing favorable numerical dissipation for linear elastic systems. However, its performance for nonlinear systems is unclear due to a lack of analytical evaluation techniques. Thus, a novel technique is proposed herein to evaluate its efficiency for application to nonlinear systems by introducing two parameters to describe the stiffness change. As a result, the numerical properties and error propagation characteristics of the γ-function explicit method for the pseudodynamic testing of a nonlinear system are analytically assessed. It is found that the upper stability limit decreases as the step degree of nonlinearity increases; and it increases as the current degree of nonlinearity increases. It is also shown that this integration method provides favorable numerical dissipation not only for linear elastic systems but also for nonlinear systems. Furthermore, error propagation analysis reveals that the numerical dissipation can effectively suppress the severe error propagation of high frequency modes while the low frequency responses are almost unaffected for both linear elastic and nonlinear systems. 相似文献
Introduction The 21st century is the time for the exploitation and utilization of underground spaces. It is astrategic way to defeat the severe challenge that human are now confronting with, such as the lackof land, contamination of environment, traffic jam, waste of sources, and so on. The exploitationand utilization of underground space has become a global tendency, and a significant sign inevaluating the speed of modernization of cities. Facing the challenge, it is significantly necess… 相似文献
Introduction The velocity field of surface plate motion can be split into a poloidal and a toroidal parts.At the Earth′s surface,the toroidal component is manifested by the existence of transform faults,and the poloidal component by the presence of convergence and divergence,i.e.spreading and subduc-tion zones.They have coupled each other and completely depicted the characteristics of plate tec-tonic motions.The mechanism of poloidal field has been studied fairly clearly which is related to … 相似文献
Zoned quartz and feldspar phenocrysts of the Upper Carboniferous eastern Erzgebirge volcano-plutonic complex were studied by cathodoluminescence and minor and trace element profiling. The results verify the suitability of quartz and feldspar phenocrysts as recorders of differentiation trends, magma mixing and recharge events, and suggest that much heterogeneity in plutonic systems may be overlooked on a whole-rock scale. Multiple resorption surfaces and zones, element concentration steps in zoned quartz (Ti) and feldspar phenocrysts (anorthite content, Ba, Sr), and plagioclase-mantled K-feldspars etc. indicate mixing of silicic magma with a more mafic magma for several magmatic phases of the eastern Erzgebirge volcano-plutonic complex. Generally, feldspar appears to be sensitive to the physicochemical changes of the melt, whereas quartz phenocrysts are more stable and can survive a longer period of evolution and final effusion of silicic magmas. The regional distribution of mixing-compatible textures suggests that magma mingling and mixing was a major process in the evolution of these late-Variscan granites and associated volcanic rocks.
Quartz phenocrysts from 14 magmatic phases of the eastern Erzgebirge volcano-plutonic complex provide information on the relative timing of different mixing processes, storage and recharge, allowing a model for the distribution of magma reservoirs in space and time. At least two levels of magma storage are envisioned: deep reservoirs between 24 and 17 km (the crystallisation level of quartz phenocrysts) and subvolcanic reservoirs between 13 and 6 km. Deflation of the shallow reservoirs during the extrusion of the Teplice rhyolites triggered the formation of the Altenberg-Teplice caldera above the eastern Erzgebirge volcano-plutonic complex. The deep magma reservoir of the Teplice rhyolite also has a genetic relationship to the younger mineralised A-type granites, as indicated by quartz phenocryst populations. The pre-caldera biotite granites and the rhyodacitic Schönfeld volcanic rocks represent temporally and spatially separate magma sources. However, the deep magma reservoir of both is assumed to have been at a depth of 24–17 km. The drastic chemical contrast between the pre-caldera Schönfeld (Westfalian B–C) and the syn-caldera Teplice (Westfalian C–D) volcanic rocks is related to the change from late-orogenic geotectonic environment to post-orogenic faulting, and is considered an important chronostratigraphic marker. 相似文献