A late pleistocene to recent rhyodacite-trachybasalt-basaltic latite volcanic association in north-east Papua |
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| Authors: | B P Ruxton |
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| Institution: | 1. Division of Land Research, Commonwealth Scientific and Industrial Research Organization, Canberra, Australia
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| Abstract: | The Managalase Plateau in north-east Papua is a faultbounded block of fractured basement metabasalt and basic plutonic rock overlain in the east by some thirty small volcanic centres of late Pleistocene to Recent age which include rhyodacite ash cones and trachybasalt and basaltic latite lava. To the north andesitic strato-volcanoes of Pleistocene and Recent age occur along the margin of the Cape Vogel geosyncline, whereas to the south a series of basaltic latite, alkali basalt and ultra-alkaline lava of Pliocene age margin the Papuan basic-ultrabasic belt. There is a transition, probably by fractional crystallization from alkali basalt through trachybasalt towards trachyandesite, but the basaltic latite and ultraalkaline lava are aberrant offshoots, possibly owing to contamination by phyllite. The « orogenic andesite » shows disequilibrium relations among the minerals including complexly zoned and twinned plagioclase, and common xenocrysts and xenoliths. Their chemistry can be simulated by mixtures of rhyodacite with either alkali basalt or trachybasalt and they are richer in both magnesia and potash than other calc-alkali rock suites. The rhyodacites, however, are of normal cale-alkali character. |
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