M.Sc. Thesis 1998 PETROLOGY AND GEOCHEMISTRY OF THE BALUNGAO GROUP OF VOLCANIC CENTERS, PANGASINAN - NUEVA ECIJA, LUZON Rolando E. Peña National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City 1101 Abstract The Balungao Group of volcanic centers - consisting of Mounts Amorong, Balungao and Cuyapo -together with Mt. Arayat, forms a volcanic belt in eastern Central Luzon. In tandem with the volcanic belt in western Central Luzon, these belts constitute the double-arc Central Luzon segment of the Luzon volcanic arc related to subduction of the south China plate along the meridional zone defined by the Manila Trench. The Balungao Group consists of medium to high K rocks with lithologies that range from basalt - basaltic andesite (Amorong) to andesite (Cuyapo) to dacite (Balungao). Moreover, both Cuyapo and Balungao have adakitic signatures in varying degrees. What is peculiar is that the degree of increasing enrichment in REE proceeds from Balungao to Cuyapo to Amorong. Harker variation diagrams and trace element ratios confirm the unique characteristics of each individual volcanic center. This leads to the conclusion that they were not derived from a parent magma that underwent fractional crystallization to produce the various rock suites, but derived rather from magmas with unique source region characteristics that were probably modified in the course of their ascent to the surface. The rocks of Mt. Arayat, which forms the other end of the eastern belt of the Central Luzon segment of the arc, have many characteristics in common with Mt. Amorong. They are both tholeiitic to calc-alkaline, medium K to high K basalts and basaltic andesites with similar REE patterns. However, Harker variation diagrams and trace element ratios indicate that they probably do not have a common parental magma. Indeed, the rocks of Arayat do not seem to have been produced from a single parent magma through crystal fractionation, but more probably represent products from magma batches that have undergone changes before being extruded. In comparison with the eastern belt (Balungao Group and Arayat), the volcanic rocks of the western belt, are less enriched in K2O, LILEs, LREEs and HFSEs at a given SiO2 and there are indications that some of the rock suites are derived through crystal fractionation from a parent magma. The dacites of Pinatubo and others in the western belt also have adakitic signatures in terms of Sr/Y ratios, but these are not as high as those of Balungao or Cuyapo. These rocks, including those of the eastern belt, could have obtained these adakitic character through crustal assimilation in the course of the ascent of the magma to the surface. The general characteristics of the volcanic rocks of the western and eastern belts of the Central Luzon segment of the Luzon arc largely conform to characteristics of subduction-related double arc systems observed in other parts of the globe. The petrogenetic model of Tatsumi and others (1986, 1992, 1994) probably also finds application for the Central Luzon double arc, whereby aqueous fluids released from the dehydration of amphibole-chlorite and phlogopite from the subducting slab at depths of ~ 100 km and ~ 200 km induce partial melting in the mantle wedge. The character of the magmas generated could be influenced by differing degrees of partial melting as well as by other processes such as magma mixing, recharge and even crustal assimilation. .