Petrology and geochemistry of volcanic rocks from the Pocdol Mountains, Bicol Arc (Philippines)

The Pocdol Mountains are part of the Bicol arc-- a SE-trending calcalkaline volcanic belt adjacent to the Philippine Trench. Recent volcano-stratigraphic studies and five new K-Ar ages have delineated seven lithostratigraphic units in the Pocdol Mountains. The rocks are grouped into : (i) a Western Pocdol Mountains (WPM) series, and (ii) an Eastern Pocdol Mountains (EPM) series. WPM eruptives comprise Early Pliocene basaltic tuff breccias and lavas (Malobago volcanics) and Middle to Late Pleistocene (0.478-0.065 Ma) andesitic lavas and tuff breccias (Lison and Kayabon volcanics). EPM volcanism produced (i) Middle Pliocene dacitic tuff breccias and minor lavas and the Matacla Dome (Suminandig volcanics). The stratified unit is intercalated with lenses of siltstones and sandstones (Rangas conglomerate), and was later (0.065 Ma) intruded by the Rangas microdiorite; (ii) Middle Pliocene to Early Pleistocene andesi tic tuff breccias, laharic breccias and lavas (Pangas volcanics), which were erupted mostly from four flank vents, with associated igneous intrusives (Pangas intrusives), and (iii) Late Pleistocene to Recent andesitic lavas, tuff breccias and lahars of the Cawayan volcanics (<0.04 Ma) and basaltic tephras and minor lavas of the Pulog volcanics (<0.03 Ma). The rocks are plagioclase-phyric, with minor clino- and orthopyroxene, titanomagnetite and hornblende. Olivine is only found in the Malobago volcanics. Glomerophyric and pilotaxitic textures are common, and most phenocrysts show normal or oscillatory zonation. Disequilibrium features are rare. The inferred crystallisation sequence of WPM lavas is titanomagnetite-olivine-pyroxene-amphibole, accompanied by plagioclase. EPM rocks have the same order of crystallisation as WPM lavas, except that olivine was not involved. Overall mineralogy of the lavas suggests a low pressure (