GEOCHEMISTRY OF LAVAS FROM TAAL VOLCANO, SOUTHWESTERN LUZON, PHILIPPINES - EVIDENCE FOR MULTIPLE MAGMA SUPPLY-SYSTEMS AND MANTLE SOURCE HETEROGENEITY

Taal Volcano is a highly active center in the southwest Luzon volcanic field. Recent eruptions are confined to Volcano Island, a plexus of cinder cones and tuff rings surrounding a central crater, occupying (and post-dating) a massive volcano-tectonic depression resulting from multiple phases of collapse. Taal lava series can be distinguished from each other by differences in major and trace element trends and trace element ratios, indicating multiple magmatic systems associated with discrete centers in time and space. On Volcano Island, contemporaneous lava series range from typically calc-alkaline to iron-enriched. Major and trace element variation in these series can bc modelled by fractionation of similar assemblages, with early fractionation of titano-magnetite in less iron-enriched series. However, phase compositional and petrographic evidence of mineral-liquid disequilibrium suggests that magma mixing played an important role in the evolution of these series. We interpret this to mean that mixing occurred along a line of descent defined by fractional crystallization, with mixing occurring over larger compositional intervals in the less iron-enriched series, consistent with numerical modelling. Between-series incompatible element ratio differences are not explicable through differential partial melting of a homogeneous source (also confirmed by variation in Pb isotopic ratios), suggesting a supply system from discrete melt zones in heterogeneous mantle.