Low-K Calc-Alkaline Signature Of Calayan Lavas (Northern Luzon, Philippines)

Ronaldo R. Pendon

Senior Science Research Specialist
Geothermal and Coal Resources Division
Department of Energy –  Republic of the Philippines
 

Abstract

The Calayan Island is a Tertiary (7-4 Ma) volcanic island situated in the northern portion of the Philippine archipelago, a part of Babuyan Segment (Babuyan and Batanes Groups of Islands) of the Luzon Arc. It is a product of subduction of South China Sea oceanic crust underneath the Philippine Terrane along the Manila Trench. Calayan has four major extinct volcanic centers namely: Mts. Nangabaywanan, Calayan, Macara, and Piddan. The rock spectrum ranges from basalt to rhyolite with andesite as the most predominant, which marked by effusive and explosive volcanic activities. Overlying these volcanics rocks in the coastline is Plio-Quaternary reefal limestones. Majority of the volcanic rocks exhibit porphyritic texture mostly in andesites, basaltic-andesites and dacites, and to lesser degree in basalts. Basalts generally show more vitrophyric texture, whereas rhyolite lavas have sub-aphyric. Phenocrysts of plagioclase are the most abundant minerals in almost the entire variation of rock types from basalt to dacite and also in lesser extent to rhyolite. Ferromagnesian minerals such as pyroxene, hornblende, biotite and olivines are also present. Geochemical signatures of these lavas are characterized by low concentrations of large-ion lithophile (LILE) and flat behavior of rare-earth elements (REE) , as significant characteristics of low-K calc-alkaline series.