Melt generation and magma storage conditions of primitive arc lavas in the Macolod Corridor, southwestern Luzon arc, Philippines

Abstract

Decoding the origin of primitive arc magmas from petrological, geochemical, and thermobarometric constraints is crucial for understanding their melt generation process and crustal storage conditions. In the Philippine arc setting, primitive basalts have been recognized in the Macolod Corridor, southwestern Luzon arc. The Macolod Corridor is a 30 by 60-km northeast-southwest striking, young rift system that hosts several Quaternary stratovolcanoes including the active Taal Volcano and Mt. Banahaw, lava domes, and ∼ 200 monogenetic centers classified as scoria cones, tuff cones, tuff rings, and maars. This study reports textural, petrological, and geochemical analyses of the Macolod primitive basalts to decipher their petrogenesis and elucidate their pre-eruptive magma storage conditions. We identified at least five distinct primitive lava compositions based on their modal mineralogy: clinopyroxene-olivine basalts, plagioclase-olivine-clinopyroxene basalts, olivine-plagioclase-clinopyroxene basalts, clinopyroxene-plagioclase-olivine basalts, and clinopyroxene-olivine-plagioclase basalts. Clinopyroxene-olivine basalts and clinopyroxene-plagioclase-olivine basalts occur as lapilli and bomb deposits. In contrast, plagioclase-olivine-clinopyroxene-basalts, olivine-plagioclase-clinopyroxene basalts, and clinopyroxene-olivine-plagioclase basalts occur as lapilli and volcanic bombs in monogenetic volcanoes and as basaltic blocky lava flows in small polygenetic volcanoes. Phenocrysts, glomerocrysts, and microphenocrysts assemblages include olivine, clinopyroxene, plagioclase ± spinel in a glassy matrix. The basalts are identified as subalkaline, medium-K, and medium-Fe tholeiitic basalts, based on their bulk-rock geochemistry. Adding 3–4 % equilibrium olivine to the Macolod primitive basalts generates magmas in equilibrium with mantle olivines with Fo_{90.68–90.82} and 0.392–0.395 wt% NiO compositions. Disequilibrium textures exhibited by olivines, clinopyroxenes, and plagioclases suggest that these are products of magma decompression and dissolution processes. Calculated melt based on olivines reveals that these primitive magmas last equilibrated at depths ranging from ∼36–42 km (1.03–1.23 GPa) at 1286°–1318 °C. Application of clinopyroxene-only thermobarometer results indicate clinopyroxene crystallization depths of around 7–16 and 10–19 km for hydrous and anhydrous estimates, respectively. The segregation depths estimated in this study translate to the uppermost mantle until near the Moho boundary whereas the storage depths correspond to prolonged magma storage regions in the upper crust as modeled by existing geophysical data (i.e., seismic travel-time tomography). Combining textural and geochemical results from this study with existing geophysical data provides new insights into the magma plumbing systems in the region and how these might operate through time.