Deep Crustal Crystallization Of Tholeiitic Melt: Insights From Manguao Basalt, Palawan, Philippines

Manguao Basalt is a Plio-Pleistocene basaltic lava flow located on the northeastern edge of Palawan Island, Philippines. The absence of active trenches surrounding the Palawan Continental Block (PCB) poses a challenge regarding the nature and origin of magmatism in the region. This study presents the petrographic and geochemical character of Manguao Basalt, as well as provides insights to the melt formation beneath the PCB. Manguao Basalt samples are olivine-phyric with minor pyroxenes, set in a plagioclase-dominated microcrystalline groundmass. Average bulk-rock major element composition of Manguao Basalt shows similarities to common olivine basalt tholeiite. Petrographic observations of the phenocrysts, however, show the unusual precedence of pyroxenes in the crystallization sequence. Calculated formation temperatures and pressures of the modal assemblage are consistent with this idea of early pyroxene formation. Simulations of mantle melting using the MELTS program show the formation of pyroxene-saturated primitive liquids. The evolution of these primitive liquids reaches similarities with Manguao Basalt composition at 1230-1260 degrees C. Simulations of equilibrium and fractional crystallization demonstrate the formation of olivine or orthopyroxene as the first crystals. However, the simulations done at equilibrium conditions are more consistent with the observed mineral chemistry of pyroxene phenocrysts in Manguao Basalt. Hence, maintaining the equilibrium between the source and melt is crucial for replicating the observed pyroxene chemistry. Magmatic underplating provides an excellent model for visualizing the melting and crystallization processes beneath the PCB. The model is also consistent with the narrative of other magmatic units in northern Palawan (e.g., Capoas Granite). The significant findings of this study contribute to the understanding of the tectonic evolution of the PCB.