Silicic Magmas Erupted From the Laguna de Bay Caldera, Macolod Corridor, Luzon, Philippines: Geochemistry and Origin

The Laguna de Bay Caldera is a depression 200 km² in diameter that occurs within the Macolod Corridor. The Macalod corridor is a NE-SW zone of rifting through the central part of Luzon that was the site of extensive Pliestocene to Holocene volcanism, including major pyroclastic eruptions from the Laguna de Bay Caldera. This caldera has erupted large volumes of pyroclastic material, with poorly constrained published ages from < 27 ka to > 50 ka. The range in pumice sample composition in these flow units is from 53 to 69 wt. % SiO₂. The abundant silicic compositions (>64 wt. % SiO₂) are the focus of this investigation. Published chemical data from two nearby and relatively young, subduction related, stratavolcanoes, Taal and Makiling, show that both also contain silicic deposits. A comparison of theses silicic deposits to the silicic samples from Laguna de Bay indicate that the Laguna de Bay pyroclastic deposits contain much higher K₂O/Na₂O (< 1 for both Taal and Makiling and >6 for Laguna de Bay). Sr concentrations in the silicic samples from Laguna de Bay are high (> 250 ppm), which precludes large amount of plagioclase fractionation. The small Eu anomaly is consistent with this interpretation. The REE element patterns for Laguna de Bay are LREE enriched with flat HREE. No depletion occurs in the middle REE. The lack of depletion in the middle REE is in contrast to a significant concave upward pattern for the Makiling samples, an indication of amphibole in the source (no REE data are available for Taal Volcano). Our preliminary conclusions are that the silicic samples from Laguna comprise distinct compositional groups, which may be interpreted as distinct magma batches. The very high K₂O/Na₂O values can be used to argue against the origin of these silicic magmas by fractional crystallization or partial melting of basaltic compositions. Melting or assimilation of a more evolved source must be involved. Evolved preexisting continental crust is absent in this area. Therefore we propose that the origin of the silicic magmas from the Laguna de Bay caldera is related to melting of preexisting subduction related, evolved crust.