@phdthesis{oai:air.repo.nii.ac.jp:00005324,
 author = {Refran, James Cesar Avisado},
 month = {Sep},
 note = {Manguao Basalt is a basalt-basaltic andesite lava flow field found on the northeast end of Palawan Island, Philippines. The relatively stable nature of the Palawan Continental Block seemingly contradicts the presence of the geomorphologically young lava flow field. This study investigates the occurrence and origin of Manguao Basalt, and its relation to the evolution of the Palawan microcontinent.
Good exposures of the Manguao Basalt lava flow field are found nearshore. Lava flow structures and features (i.e., levees, channels, and tumuli) are still intact and observable in several locations. The lava flow is generally blocky and vesicular, with observable phenocrysts and large chert xenoliths. Macroscopic observations of obtained samples show identifiable olivine and pyroxene phenocrysts. Petrographic observations show olivine dominance in the phenocryst assemblage and plagioclase dominance in the groundmass. Pyroxenes are found both in phenocryst and groundmass assemblages. Mineral textures observed from petrography show the earlier formation of pyroxene phenocrysts from olivine. Plagioclase laths, on the other hand, show earlier crystallization from the anhedral groundmass pyroxenes. Volcanic glass fills the interstices between plagioclase and pyroxene, forming intersertal textures. A general overview of the crystallization sequence (pyroxenes → olivine → plagioclase → groundmass pyroxenes) is presented.
Major element chemistry from bulk-rock compositions characterizes the Manguao Basalt as subalkaline and tholeiitic. Bivariate diagrams show the fractionation of ferromagnesian minerals (i.e., olivine and pyroxenes) but limited fractionation in later-formed minerals (i.e., plagioclase). Total iron over magnesium (FeOt/MgO) versus silica (SiO2) plots (Miyashiro, 1974) of Manguao Basalt shows 'straddling' tholeiite and calc-alkaline character. The presence of the chert xenoliths, however, may have affected bulk silica concentrations, shifting the plots towards the calc-alkaline field. AFM ternary plots show a diagnostic early iron enrichment, reflecting a tholeiitic trend. Trace element chemistry confirms the early ferromagnesian fractionation from nickel and cobalt trends. Tectonic discrimination diagrams of Manguao Basalt show resemblance with different basalts from other known tectonic settings (i.e., continental flood basalts, island-arc basalts, continental-arc basalts, and within-plate basalts). However, the association of Manguao Basalt with ocean-island basalts (OIB) or mid-ocean ridge basalts (MORB) appeared inconclusive [i.e., plotting outside MORB-OIB array defined by Pearce (2008)]. These inconsistencies that accompany the trace element discrimination diagrams have been pointed out by Li et al. (2015). Nevertheless, their utility is in constraining the possible tectonic processes that influence melt generation and evolution. Trace element patterns in spider diagrams show the enriched melt character of Manguao Basalt. Characteristic Nb-Ta depletion is absent for Manguao Basalt, indicating source melting that is unrelated to subduction. Positive anomalies in continent-derived trace elements (e.g., Th and Pb) are observable and consistent in any spider diagram presented (Primitive Mantle-, MORB-, NMORB-, EMORB-, and OIB-normalized).
Mineral chemistry data (olivine, orthopyroxene, clinopyroxene, and plagioclase) confirm the crystallization sequence inferred from petrography. The highest Mg# was obtained from orthopyroxene phenocrysts (Mg# > 80). Calculations using orthopyroxene barometer also yielded the highest pressures (~5 kbar), while orthopyroxene thermometers (i.e., opx-liquid) yielded the highest temperatures (~1260 °C). These obtained pressure and temperature conditions were used as benchmarks for simulations of source melting and crystallization.
Melting simulations of different upper mantle sources (e.g., fertile and depleted) were done using the MELTS program. Equilibrium and fractional melting from depressurization (15 kbar → 5 kbar; T = 1350 °C and at QFM buffer) of any mantle source produce liquids that are saturated in pyroxene components. Subsequent crystallization of melt, while being at equilibrium with source and at elevated pressures (~5 kbar) produce orthopyroxenes with similar compositions to orthopyroxenes in Manguao Basalt. The formation of olivine and the rest of the groundmass cannot be reconciled with any of the simulations due to the assumption of complete crystallization at depth. The model for magmatic underplating is presented here as a possible setup for the origin of Manguao Basalt.},
 school = {秋田大学},
 title = {Descriptive Petrology and Geochemical Study of New Basalt Volcanic Activity in Palawan Island, Northern Province, Philippines},
 year = {2020}
}