Leo, G.W., Hedge, C.E. and Marvin, R.F., 1980. Geochemistry, strontium isotope data, and potassium-argon ages of the andesite-rhyolite association in the Padang area, West Sumatra. J. Volcanol. Geotherm. Res., 7: 139-156.

Quaternary volcanoes in the Padang area on the west coast of Sumatra have produced two-pyroxene, calc-alkaline andesite and volumetrically subordinate rhyolitic and ande• sitic ash-flow tuffs. A sequence of andesite (pre-caldera), rhyolitic tuff and andesitic tuff, in decreasingorder of age, is related to Maninjau caldera.....Andesite compositions range from 55.0 to 61.2%Si02 and from 1.13 to 2.05% K20. Six K-Ar whole-rock age deter• minations on andesites show a range of 0.27 ± 0.12 to 0.83 ± 0.42 m.y.; a single determi• nation on the rhyolitic ashflow tuff gave 0.28 ± 0.12 m.y.
Eight 07Sr/16Sr 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 vol•
canics of continental margins. An 87Sr/86Sr ratio of 0. 7048 on G. Sirabungan east of the
Sernangko fault is similar to an earlier determination on nearby G. Marapi (0.7047), and agrees with 87Sr/80Sr ratios in the rest of the Sunda arc. The reason for this distribution of
87Sr/86Sr ratios is unknown.
The high 87Sr/"6Sr 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 plagioclasewith hornblende and/or pyroxenes. Generation of associated andesite and rhyolite could have been caused by hydrous frac• tional melting of andesite or volcanogenic sediments under adiabatic decompression.

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