The Mt. Bulusan Story:  From SAR Imagery to Isotope Geochemistry

Francisco G. Delfin Jr.¹, Frank McDermott², Marc J. Defant³, Simon Turner⁴, and Rene Maury⁵

¹School of Policy, Planning, and Development, University of Southern California
²University College Dublin
³University of South Florida, Tampa
⁴Macquarie University, Sydney
⁵Universite Bretagne Occidentalé, Brest (France)

Abstract

A petrogenetic story of the Bulusan volcanic complex (BVC) in the Bicol arc is presented by integrating data from SAR imagery, geologic mapping, geochronology, petrology, whole-rock chemistry, and isotope geochemistry. Two volcanic series are present in the BVC: a high-K series constructed from 2.14 Ma to 1.56 Ma and a medium-K series formed during successive pre- (at least 1.10 Ma), syn- (35-40 Ka), and post-caldera eruptions. The 11-13 km-wide Irosin caldera, the largest known Quaternary silicic ash-flow caldera in the Philippines, formed about 35-40 Ka when hot basaltic magma was recharged into a silicic chamber triggering a cataclysmic eruption of rhyolitic pyroclastic flows. After caldera collapse, portions of the triggering basalt were extruded forming Sharp Peak volcano. But later post-caldera volcanism is dominantly two-pyroxene andesite in composition.

Trace element and isotope geochemistry belies Divis’ (1980) contention that Bulusan and other Bicol arc andesites are primary magmas generated by partial melting of the subducted Philippine Sea plate. Instead, these andesites are fractionated from parental basaltic magmas produced by melting of a mantle-wedge source. The Bicol arc source region contrasts markedly with those of the West Luzon arc based on the former’s lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd ratios. ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb isotope data also suggest that Bulusan (and the Bicol arc’s) pre-subduction mantle wedge is similar to that of the Palau-Kyushu ridge east of the Philippine Trench but modified by subducted pelagic sediments incorporated into the wedge as a melt.