The petrogenesis of volcanics from Mt. Bulusan and Mt. Mayon in the Bicol arc,the Philippines |
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| Authors: | Email author" target="_blank">Frank?McDermottEmail author Francisco?G?Delfin?Jr Marc?J?Defant Simon?Turner Rene?Maury |
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| Institution: | (1) UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland;(2) School of Policy, Planning, and Development, University of Southern California, Los Angeles, CA 90089, USA;(3) Department of Geology, University of South Florida, Tampa, FL 33620, USA;(4) Department of Earth and Planetary Sciences, Macquarie University, 2109 Sydney, NSW, Australia;(5) CNRS-UMR 6538, IUEM, Universite Bretagne Occidentale, 29280 Plouzane, France |
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| Abstract: | Pliocene to recent volcanic rocks from the Bulusan volcanic complex in the southern part of the Bicol arc (Philippines) exhibit
a wide compositional range (medium- to high-K basaltic-andesites, andesites and a dacite/rhyolite suite), but are characterised
by large ion lithophile element enrichments and HFS element depletions typical of subduction-related rocks. Field, petrographic
and geochemical data indicate that the more silicic syn- and post-caldera magmas have been influenced by intracrustal processes
such as magma mixing and fractional crystallisation. However, the available data indicate that the Bicol rocks as a group
exhibit relatively lower and less variable 87Sr/86Sr ratios (0.7036–0.7039) compared with many of the other subduction-related volcanics from the Philippine archipelago. The
Pb isotope ratios of the Bicol volcanics appear to be unlike those of other Philippine arc segments. They typically plot within
and below the data field for the Philippine Sea Basin on 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, implying a pre-subduction mantle wedge similar to that sampled by the Palau Kyushu Ridge, east of the Philippine
Trench. 143Nd/144Nd ratios are moderately variable (0.51285–0.51300). Low silica (<55 wt%) samples that have lower 143Nd/144Nd tend to have high Th/Nd, high Th/Nb, and moderately low Ce/Ce* ratios. Unlike some other arc segments in the Philippines
(e.g. the Babuyan-Taiwan segment), there is little evidence for the involvement of subducted terrigenous sediment. Instead,
the moderately low 143Nd/144Nd ratios in some of the Bicol volcanics may result from subduction of pelagic sediment (low Ce/Ce*, high Th/Nd, and high
Th/Nb) and its incorporation into the mantle wedge via a slab-derived partial melt. |
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