| Abstract: | Quaternary volcanoes in the Padang area on the west coast of Sumatra have produced two-pyroxene, calc-alkaline andesite and volumetrically subordinate rhyolitic and andesitic ash-flow tuffs. A sequence of andesite (pre-caldera), rhyolitic tuff and andesitic tuff, in decreasing order of age, is related to Maninjau caldera. Andesite compositions range from 55.0 to 61.2% SiO2 and from 1.13 to 2.05% K2O. Six K-Ar whole-rock age determinations on andesites show a range of 0.27 ?? 0.12 to 0.83 ?? 0.42 m.y.; a single determination on the rhyolitic ashflow tuff gave 0.28 ?? 0.12 m.y. Eight 57Sr/26Sr ratios on andesites and rhyolite tuff west of the Semangko fault zone are in the range 0.7056 - 0.7066. These ratios are higher than those elsewhere in the Sunda arc but are comparable to the Taupo volcanic zone of New Zealand and calc-alkaline volcanics of continental margins. An 87Sr/86Sr ratio of 0.7048 on G. Sirabungan east of the Semangko fault is similar to an earlier determination on nearby G. Marapi (0.7047), and agrees with 87Sr/86Sr ratios in the rest of the Sunda arc. The reason for this distribution of 87Sr/86Sr ratios is unknown. The high 87Sr/86Sr ratios are tentatively regarded to reflect a crustal source for the andesites, while moderately fractionated REE patterns with pronounced negative Eu anomalies suggest a residue enriched in plagioclase with hornblende and/or pyroxenes. Generation of associated andesite and rhyolite could have been caused by hydrous fractional melting of andesite or volcanogenic sediments under adiabatic decompression. ?? 1980. |
| Genre: | Article |
| ProdID: | 70012313 |
| Citation Author: | Leo, G. W.; Hedge, C. E.; Marvin, R. F. |
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| Citation End Page: | 156 |
| Citation Issue: | 1-2 |
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| Citation Language: | English |
| Citation Larger Work Title: | Journal of Volcanology and Geothermal Research |
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| Citation Number Of Pages: | 18 |
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| Citation Search Results Text: | Geochemistry, strontium isotope data, and potassium-argon ages of the andesite-rhyolite association in the Padang area, West Sumatra; 1980; Article; Journal; Journal of Volcanology and Geothermal Research; Leo, G. W.; Hedge, C. E.; Marvin, R. F. |
| Citation Start Page: | 139 |
| Citation Volume: | 7 |
| Citation Year: | 1980 |
| Type: | citation/reference |
| Text: | Geochemistry, strontium isotope data, and potassium-argon ages of the andesite-rhyolite association in the Padang area, West Sumatra; 1980; Article; Journal; Journal of Volcanology and Geothermal Research; Leo, G. W.; Hedge, C. E.; Marvin, R. F. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Tue, 1 Jan 1980 00:00 -0600 |
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