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

Abstract

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 (<9 kb) crystallisation and estimated equilibration temperatures from coexisting two pyroxenes range from 1006°C to 1135°C. The absence of ilmenite phases precludes an estimate of oxygen fugacity. WPM lavas comprise medium-K high-Al basalt to medium-K and high-K andesite, whereas the EPM rocks consist of low-K basaltic andesite to medium-K andesite and dacite. Major oxide and trace element variations indicate two possible parental liquids, each generating the WPM and EPM series; the EPM lavas also show two fractionation trends : a dacite and an andesite crystallisation paths. WPM lavas generally contain greater abundances of large-ion lithophile (LIL) and high field strength (HFS) ions, but they have lower concentrations of ferromagnesian elements. Low Mg/(Mg+Fe²), Ni and Cr values in both series suggest that the liquids are not in equilibrium with mantle peridotite. Both WPM and EPM lavas are considered to have been derived by closed-system low pressure POAM fractionation (Gill, 1981) of a basaltic source, that may have been generated by higher degrees of partial melting within the mantle wedge and/or the subducted slab, together with some degree of enrichment from the downgoing slab. Stratigraphic criteria and least-squares mixing models indicate that by precipitating plagioclase, orthopyroxene, titanomagnetite and clinopyroxene, both Lison and Kayabon andesites (WPM series) were probably derived from a high-alumina basaltic source (Malobago volcanics), whereas the Suminandig, Pangas and Cawayan volcanics (EPM series) originate from a low-K basaltic andesite liquid (Pulog volcanics). However, if it is assumed that a dacitic melt was sitting on top of the EPM reservoir, then it is necessary to invoke liquid fractionation (McBirney et al., 1985), whereby the more fractionated liquids move upward and are collected at the roof of the chamber, due to density stratification in the magma reservoir.