Petrologic and Geochemical Nature of the Uppermost Mantle beneath Mt. Pinatubo, Philippines: Constraints from Mantle Peridotite Xenoliths

English Abstract:

Infamous for its Plinian eruption on June 15, 1991, Mt. Pinatubo is a stratovolcano western Luzon with its magmatic activities being attributed to the eastward subduction of the South China Sea Plate along the Manila Trench (e.g. Newhall et al., 1996). Aside from spewing voluminous amounts of ash and gases, magmatism in Mt. Pinatubo has transported mantle peridotite xenoliths to the surface hosted by dacite rocks near the volcano’s crater. A total of 52 samples were collected during our fieldwork with harzburgite as the most abundant lithology. A single spinel lherzolite (P12-7) was also observed along with minor amounts of dunite, pyroxenite and websterite. The harzburgites dominantly show porphyroclastic texture and are composed of olivine, orthopyroxene and spinel. Some portions of the harzburgite are already metasomatized as manifested by the occurrence of secondary orthopyroxene and amphiboles. The rare spinel lherzolite (P12-7) is significantly fresh and unaltered. It is also porphyroclastic and composed of coarse grains of olivine, clinopyroxene, orthopyroxene and chromian spinel. Kink banding and micro-inclusion trails are easily observed in the olivine grains. Orthopyroxenes are large porphyroclasts with exsolution lamellae. Clinopyroxenes are subhedral to anhedral in shape and are turbid in appearance. Vermicular and reddish brown chromian spinels are also disseminated within the sample. Chromian spinel in the lherzolite have low Cr# (=0.24-0.38) compared to the harzburgites (Cr#=0.28-0.50). A plot of Fo content vs the spinel Cr # shows a higher Fo content (=91-92) than the harzburgite (=90.5-91). This indicates that the lherzolite may possibly represent the intact and unaltered suboceanic type mantle beneath Mt. Pinatubo whereas the harzburgites are typical of a metasomatized subarc mantle.