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1.
Hydrous Basaltic Andesites Associated with Minette and Related Lavas in Western Mexico 总被引：3，自引：1，他引：3
The convergent margin of western Mexico is uniquely characterizedby a volcanic front of lamprophyric and related lavas located{small tilde}70 km closer to the Middle America trench thanthe main axis of andesitic volcanism. This front, defined bysmall volcanic centers ranging in age from {small tilde}1 kato 3 Ma, contains several lava types: minette, absarokite, leucitite,spessartite, and kersantite, all without feldspar phenocrysts.Many of the lavas contain hydrous phenocrysts; they are enrichedin potassium and other incompatible elements, and they are moreoxidized relative to the andesitic suite of the main axis. Intimatelyassociated are flows of basaltic andesite of comparable volume.They range in composition from 53 to 58 wt.%SiO2, have 5–9wt.%MgO and contain phenocrysts of olivine, sparse augite, andvarying amounts of plagioclase. Their alkali contents are typicalof calc-alkaline varieties, with average Na2O and K2O concentrationsof 4?2 and 1?1 wt.% respectively. The basaltic andesites oftencontain olivine of unusually high forsterite content, reflectingcrystallization under oxidizing conditions, and they have oxygenfugacities up to 3?3 log units above the Ni-NiO buffer. Manifestationsof high water contents are (1) the ubiquitous occurrence ofgroundmass olivine rather than orthopyroxene, and (2) the suppressionof plagioclase as an early crystallizing phase. Both featuresreflect the role of water in reducing the activity of silicain the melt. The progressive influence of water during crystallizationis also seen in the continuum between the two intermediate lavatypes, basaltic andesite and kersantite, as plagioclase is suppressedand hornblende is stabilized in the phenocryst assemblage. Thus,despite the absence of hydrous minerals in the basaltic andesites,their phenocryst assemblages reveal the influence of substantialamounts of water, and thereby show a genetic link to the variouslamprophyric lavas. 相似文献
2.
The coexisting pyroxenes and olivines from nine British andIcelandic Tertiary porphyritic acid glasses have been separatedand analysed chemically. Eight new analyses of augite and ferroaugites,one of orthopyroxene, and five of iron-rich olivines are presented,together with their optical properties. The trend of crystallizationof the ferroaugites is discussed. New optical determinativecurves for these ferroaugites and the iron-rich olivines arepresented. The relationship of the pyroxenes and olivines totheir analysed residual glasses is considered. 相似文献
3.
The coexisting pyroxenes and olivines from nine British andIcelandic Tertiary porphyritic acid glasses have been separatedand analysed chemically. Eight new analyses of augite and ferroaugites,one of orthopyroxene, and five of iron-rich olivines are presented,together with their optical properties. The trend of crystallizationof the ferroaugites is discussed. New optical determinativecurves for these ferroaugites and the iron-rich olivines arepresented. The relationship of the pyroxenes and olivines totheir analysed residual glasses is considered. 相似文献
4.
Pelitic and calcareous rocks in the Whetstone Lake area havean unusually wide range of chemical composition. Metamorphicreactions have been deduced that represent the observed ‘discontinuities’in compatible mineral assemblages, and by plotting the reactantand the product assemblage of each reaction on a map, metamorphicisograds have been delincated ‘from both sides’.For the pelitic rocks, successively higher-grade isograds arebased on the following reactions: (1)chlorite+muscovite+garnetstaurolite+biotite+quartz+water; (2) chlorite+muscovite+staurolite+quartz kyanite+biotite+water; (3) kyanitesillimanite; (4)staurolite+museovite+quartzsillimanite+garnet+biotite+water. A fifth isograd, based on the reaction (5) biotite+calcite+quartzCa-amphibole+K-feldspar+carbon dioxide+water intersects the isograds based on reactions (2), (3), and (4)in such a manner as to indicate that the H2O/CO2 fugacity ratiowas significantly higher in the vicinity of a granite plutonthan in the metasedimentary rocks remote from the pluton. Chemicalanalyses of the coexisting minerals in reaction (5) indicatethat the real reaction may involve plagioclase, epidote, sphene,and Fe-Ti oxides as well. 相似文献
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Textural and chemical relationships in the Ca-amphibole serieshave been examined in progressively metamorphosed mafic schistfrom the Povungnituk Group, northern Qubec. In the hornblendezone, coexisting actinolite and hornblende are characterizedby sharp optical and chemical boundaries. They also displayan epitaxial intergrowth texture, consistent with contemporaneouscrystallization and the presence of a miscibility gap (solvus)between actinolite and hornblende. At higher grade, in the oligoclasezone, actinolite is always rimmed by hornblende. Across theoligoclase isograd, the compositional break between actinoliteand hornblende remains relatively constant in width (6–10wt.% Al2O3). Accordingly, the concentric texture and the lackof chemical gradients are interpreted as a local equilibriumat the scale of the interfaces between the two amphiboles. Throughoutthe hornblende zone and the lower part of the oligoclase zone,both amphiboles are characterized by a progressive Mg-enrichment,with no consistent change in edenite and Tschermak components.The hornblende zone can be characterized by a continuous reactionwith Mg-enrichment of reactants (biotite and chlorite) and products(actinolite and hornblende). However, this reaction requiresa mixed-volatile phase with a much higher XCO2 ({small tilde}0.5) than those calculated from equilibrium curves in the NCMASH—CO2system. The oligoclase isograd can be represented by a discontinuousoligoclase—hornblende-forming reaction which coincideswith the change from epitaxial to concentric texture at theoligoclase isograd. At the upper limit of the oligoclase zone,gradational boundaries and a continuous solid solution betweenhornblende and actinolite document a disequilibrium texture.Although a miscibility gap can be documented in the hornblendezone, its closure at higher grade is not observed, based onequilibrium crystallization of actinolite and hornblende. Theresults of this study suggest that the crest of the solvus mustbe crossed between the oligoclase and actinolite-out isograds,based on zoned profiles between hornblende rims and relic actinolite. "The term exsolution refers to the process whereby an initiallyhomogeneous solid solution separates into two (or possibly more)distinct crystalline minerals without the addition or removalof material to or from the system. Exsolution generally, althoughnot necessarily, occurs on cooling. The miscibility gap representsa temperature—composition field in which solid solutionbetween the end members decreases gradually from higher to lowertemperatures." 相似文献
7.
The Tertiary flood-basalt sequence of eastern Iceland is intermittentlydisturbed by central volcanic activity with the voluminous eruptionof acid magma. Associated with one of these central volcanoes,described in this paper, is an intense swarm of acid and basicdykes, a set of acid cone-sheets, and extensive superimposedhydrothermal alteration. The lavas and intrusions which makeup the volcano grade in composition from olivine-tholeiites,through olivine-free tholeiites, basaltic-andesites, and andesites(icelandites), to rhyolites. This series is unusually rich iniron, titanium, and manganese, and poor in magnesium; aluminaand total alkalis also tend to be low in the basic members.Magnetite (sensu lato) plays a varied role in the order of crystallization,and it is only in the intermediate stages of this fractionatedseries that magnetite is available for crystal fractionation.The otherwise progressive enrichment of iron relative to magnesiumthroughout the successive liquids of the series is halted duringan intermediate stage, as magnetite becomes an early-crystallizingphase. 相似文献
8.
Abstract Mineral equilibria in the system CaO–MgO–Al2O3–SiO2–H2O provide a basis for mapping of four reaction isograds and one bathograd in the low-pressure transition from subgreenschist to greenschist facies. Most of the Matachewan area of the Abitibi greenstone belt is in the lower-pressure bathozone, as indicated by the widespread occurrences of the subassemblage Prh–Chl. The higher-pressure bathozone is indicated by two occurrences of Pmp–Act–Ep–Qtz, but in these samples the bathograd is displaced to anomalously low pressure by the high Fe content of the coexisting minerals. This illustrates the need to analyse coexisting minerals, calculate activities of end-member species, and compute P–T curves for individual samples before interpreting the isograd/bathograd pattern. Petrographic and microprobe analysis indicates that great care must be taken in the selection of ‘equilibrium’ assemblages. Pyroxene phenocrysts in one sample are replaced by the assemblage Pmp–Act–Ep–Chl–Qtz, whereas Prh–Act–Ep–Chl–Qtz occurs in the groundmass. Compositional variation may be more cryptic, as in a sample of metabasaltic hyaloclastite that contains two spatially distinct ‘univariant’ assemblages, Prh–Pmp–Ep–Chl–Qtz and Prh–Act–Ep–Chl–Qtz, within the devitrified matrix. Whereas chlorite compositions are similar in both assemblages, prehnite and epidote in the latter assemblage are significantly richer in Fe and poorer in Al. Accordingly, the rock is interpreted to contain two distinct ‘univariant’ assemblages, rather than one ‘invariant’ assemblage (Prh–Pmp–Act–Ep–Chl–Qtz). The displaced ‘univariant’ curves for this sample intersect at 2.2 kbar and 250°C. Taking account of all thermobarometric implications, the low-grade limit of the greenschist facies is at 250–270°C and 2–2.5 kbar, corresponding to depths of 7–8 km. Comparison of apparent P–T conditions on both sides of the Larder Lake – Cadillac break, a regional CO2-metasomatized fault zone that is spatially associated with many Archaean gold deposits, provides an upper limit of not more than c. 1 km for post-metamorphic south-side-up, dip-slip displacement. 相似文献
9.
A Hornblende Basalt from Western Mexico: Water-saturated Phase Relations Constrain a Pressure-Temperature Window of Eruptibility 总被引：13，自引：1，他引：13
Trachybasalt scoria from a cinder cone near the Mexican volcanicfront contain phenocrysts of olivine with chromite inclusions,apatite, augite and hornblende, with microphenocrysts of plagioclase.The water-saturated phase relations reproduce the phenocrystassemblage between 1040°C and 970°C with water contentsof between 2·5 and 4·5% (50–150 MPa). Theabsence of biotite phenocrysts in the scoria places a tightconstraint on the pressure–temperature conditions of phenocrystequilibration, as there is only a small zone where biotite doesnot accompany hornblende in the experiments. Diluting the fluidphase with CO2 changes the composition of the olivine, indicatingthat CO2 was only a minor component of the fluid of the scoria.Hornblende is stable to 1040°C at oxygen fugacities of NNO+ 2 (where NNO is the nickel–nickel oxide buffer), butat lower oxygen fugacities, the upper limit is 990°C. Thereis a progressive increase in crystallinity in experimental runsas both pressure and temperature decrease. Isobaric plots ofcrystallinity show that the onset of hornblende crystallizationinvolves a reaction relation, and also results in a marked 相似文献
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