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Volcans en Luzon, Philippines - informations / VolcanoDiscovery
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Your email only if you want to be contacted back. Send Feedback. E-mail the story Major volcanic eruption feared in Philippines. Your friend's email. English edition Main.argo-karaganda.kz/scripts/nyxiryqi/3918.php
So far, the volcano has generated clouds of smoke and spewed ash. The authorities have also imposed restrictions on flights flying in and out of the area. Content providers. Contact EFE. Quartz xenocrysts and anhydrite comprise up to 1. The fine-grained matrix is composed of clinopyroxene, plagioclase, and Fe-Ti microphenocrysts with abundant gray to light-brown matrix glass. Similar to the dacites, the andesitic groundmass contains fragments of phenocrysts.
Basalt has been found as inclusions of variable size millimeters to several centimeters in andesite. However, unlike in lamprophyres, the amphibole is not an early phase but postdates olivine and clinopyroxene. While clinopyroxene forms well-developed, euhedral crystals, the olivine is always marginally replaced by amphibole. Olivine usually contains tiny chromite inclusions. Pyrrhotite occurs as inclusions in the silicates Hattori, this volume. The matrix of the mafic minerals is formed by small laths of plagioclase with brown glass and cavities in the interstices.
The basalt also contains several large crystals which show signs of instability and may be either xenocrysts or are derived from a small amount of evolved dacitic? One of these phases is mottled, sieve-textured euhedral plagioclase. Some samples also contain quartz crystals rimmed by clinopyroxene coronas fig. Commonly, relatively large glass patches are associated with the quartz xenocrysts. Furthermore, there are decomposed mafic minerals. Textural evidence suggests that they are amphiboles that have been replaced by clinopyroxene, and perhaps also clinopyroxenes.
The contacts between basalt and andesite are generally sharp fig. Cuspate and interfingering contact relations between the two suggest that both rocks were partially molten when they came into contact. In detail, the plagioclase phenocrysts show complex zoning see Pallister and others, this volume.
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Plagioclase in evolved dacites of Mount St. Helens have similar compositions Smith and Leeman, , but those in dacites from northern Luzon are less albite rich and show much greater compositional variability within single samples Knittel and others, unpub. Figure 4. Squares, phenocryst-poor dacites; circles, phenocryst-rich dacites. With 6. The amphibole in the andesite appears to be even slightly higher in Si though this could be a sample bias; only one andesite was investigated.
The high Si content is typical for amphibole in evolved rocks. Figure 5.
Silicon versus calcium content for amphiboles number of atoms on the basis of 23 oxygen atoms. Diamonds, basalt; open squares, andesite; filled squares, dacite. They are magnesian salites generally low in titanium Ti and sodium Na table 4. Table 4.
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Microprobe analytical compositions of pyroxene. Table 5. Microprobe analytical compositions of anhydrite. The highest values are probably due to the fact that this biotite replaces Mg-rich olivine without significant addition or loss of Fe and Mg. Table 6. Microprobe analytical compositions of biotite. Xenocrystic olivines are also found in several samples of dacitic pumice.
Petrographic features of the olivines have already been described. Regardless of the host rock, olivine core compositions are uniformly Fo and rims have Fo table 7. Because the olivine grains commonly are replaced by amphibole along the margins and some grains in dacite appear to have experienced some kind of abrasion or resorption as indicated by their subhedral shape, the "original" rims may have been more Fe rich.
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Table 7. Microprobe analytical compositions of olivine. This is suggested by the observation that the most highly oxidized chromite filled stars in fig. Table 8. Microprobe analytical compositions of chromite inclusions in olivine. Figure 6. Chromite inclusions in olivine from basalt are shown as filled squares Pt42 and circles Pt Those from andesite are shown by open squares and circles two different parts of sample Pt42 , and those from dacite as filled Pt21 and open Pt9 stars.
Knittel, unpub. Clasts of both types of dacite have virtually identical compositions, with Al 2 O 3 is slightly higher in the phenocryst-poor dacite With regard to all other major and trace element abundances, the results obtained in the two laboratories are compatible within analytical uncertainty. Sulfur content in the dacite ranges from 1, to 2, ppm. The only andesite analyzed for sulfur Pin6 is slightly enriched in sulfur relative to the dacites. Two small clasts cm in diameter collected in September Pin9 and Pin10, with and ppm S, respectively clearly show the effect of leaching of anhydrite by rainwater during the first rainy season following the eruption.
Harker variation diagrams of selected major and trace elements of the Pinatubo rocks are shown in figures 7 and 8. In these figures, the Pinatubo compositions are compared to other volcanics from the Luzon arc reported by Defant and Defant and others The dacites of Pinatubo display the same geochemical characteristics that Defant and others observed for the Bataan arc-front volcanoes segment: significantly lower K 2 O values than the volcanics from the Bataan behind-arc volcanoes, Macolod, and Mindoro segments.
Figure 7. Major elements plotted against SiO 2 , for Pinatubo and other eruption products. Crosses, Pinatubo dacite, andesite, and basalt together ; diamond, Bataan behind-arc volcano lavas; square, Bataan arc-front volcano lavas; circle, Macolod corridor lavas. Figure 8. Trace elements plotted against SiO 2 , for Pinatubo and other eruption products. Symbols as in figure 7.