THE GEOCHEMISTRY AND TECTONIC SETTING OF THE NORTHERN SECTION OF THE LUZON ARC (THE PHILIPPINES AND TAIWAN)

The Luzon arc consists of a 1200 km chain of stratovolcanoes and volcanic necks stretching from Mindoro (13° N) to the Coastal Range of Taiwan (24° N). This study is concerned with three of the five major segments along the arc: the Northern Luzon, Babuyan, and Taiwan segments. The late Tertiary to Quaternary volcanics of these segments are primarily andesitic but range in composition from basalt to rhyolite and are typical arc volcanics: porphyritic plagioclase textures, primarily calc-alkaline with a few tholeiitic volcanic centers, low TiO2 concentrations, and low high-field strength element (HFSE) to large-ion lithophile element (LILE) ratios. There is a large range in K2O and other LILEs from low-K tholeiites to high-K calc-alkaline suites. Calayan island and Mt. Tabungon (volcanic substratum of Mt. Cagua) volcanics in the Babuyan segment and most rocks from the Taiwan Coastal Range are low-K tholeiites (e.g., nearly flat REE patterns, low LREE and LILE concentrations, and low Th/U ratios). The dominant calc-alkaline series ranges from medium-K to high-K rocks. The high-K calc-alkaline rocks are mainly young and are found on Batan and Lutao islands. When the low-K tholeiitic and medium- and high-K calc-alkaline rocks are associated in the same region, the low-K rocks are usually older. There is a general relationship between K2O (and other LILE concentrations) and K-Ar radiometric dates. To some extent, 87Sr 86Sr ratios also increase with time throughout the arc. There is a correlation between latitude and 87Sr 86Sr, and a positive correlation between 87Sr 86Sr and [La Sm]CN. An increase in both these geochemical parameters has been associated with an increase in the input of continental crustal material in other arc regions. The lowest 87Sr 86Sr ratios found along the entire Luzon arc (the samples from Baguio) are equivalent to those found in samples from mid-Tertiary plutons from Northern Luzon and the Bicol arc. Both groups of rocks are associated with westward subduction along the Philippine Trench where presumably deep oceanic sediments with little or no continental crustal component have been subducted. The absence of continental crust below the arc in the Northern Luzon and Babuyan segments has been suggested by several groups of researchers. This, together with the fact that metasomatized ultramafic nodules (probably of mantle origin) in disequilibrium with their Batan host lavas (but with similar 87Sr 86Sr and [La Sm]CN ratios to the host rocks), suggests that upper level assimilation (AFC processes) does not appear to be the major influence on the geochemical signatures. A collision zone between the upper crustal block of Eastern China and Taiwan and the Manila Trench has been recorded in the northern section of the arc (Taiwan). Sediments have also been shown to decrease in thickness along the South China Sea basin nearly parallel to the Manila Trench from north to south. The sediment source region is most probably Taiwan, and perhaps Eastern China. This latitudinal in variation sediment thickness may explain the crustal signature related to both geochemical and age parameters. We suggest that this crustal input has taken place via subduction of sediment rich in a crustal component. © 1990.