Abstract
Mt. Popa is situated on a line of widely-spaced, calc-alkaline volcanic centres in the central lowlands of Burma. Latites, rhyodacites, and ignimbrites are interbedded with strongly-folded arenaceous sediments of topmost Miocene and Pliocene age. These are overlain unconformably by a composite cone of basalt and basaltic andesite lavas and pyroclastic deposits of Pleistocene or early Recent age.
The two groups of lavas are regarded as a single petrographic suite in which several continuous and discontinuous mineral reaction series are recognized. Phenocryst phases, analysed by microprobe, are: olivine, bronzite-hypersthene, biotite, diopsidic augite-salite, magnesio-hastingsite-magnesio-hornblende, titaniferous magnetite, bytownite-oligoclase, and sanidine. Whole-rock analyses are characterized by relatively high K, Ba, Sr, Rb, and Zr. High Mg/(Mg + Fe") ratios of 0.85 to 0.65 in both phenocrysts and whole-rock, high K/Rb (767) and low Sr87/Sr86 (0.70431) and Rb/Sr (0.0155) ratios suggest that the basalts are relatively primitive mantle-derived compositions, modified slightly by fractionation, with negligible crustal contamination. The more siliceous lavas may have been derived by fractional crystallization of such magmas at crustal levels.
This high-K calc-alkaline suite is typical of a continental margin orogenic environment. Analyses of contemporaneous lavas from elsewhere in Burma define three N-S volcanic lines which become more alkaline from W to E. The magmatism is attributed to the waning stages of orogeny as eastward subduction of the Indian plate beneath China gave way to strike-slip movement, associated with the Andaman Sea spreading-centre. Within this tectonic model, magmas were probably generated by hydrous mantle melting above dehydrating oceanic crust of the subduction zone.
