Magmatic and amagmatic contributions to crustal growth in the Philippine island arc system: Comparison of the Cretaceous and post-Cretaceous periods

摘要

There have been few studies on both magmatic and amagmatic processes that contribute to crustal thickening. As a result, estimates of the rates at which these processes have taken place are rather limited. With the recent availability of arc magmatic addition and ophiolite accretion rates from geochemical and geophysical data, it is now possible to look at the variations of these rates in time and space especially with respect to the Cretaceous period. Arc magmatic addition rates in the Philippines vary from 25 to 60 km3/km/m.y. whereas accretion rates for the different ophiolite complexes range from 2 to 30 km3/km/m.y. Low accretion rates by arc magmatism and ophiolite accretion (<25 km3/km/m.y. and <10 km3/km/m.y., respectively) are obtained for the Cretaceous compared to post-Cretaceous rates. Moreover, lower ophiolite accretion rates are obtained for the eastern ophiolites whereas the reverse is noted for the western ophiolites (e.g., Zambales, Mindoro, Antique, Palawan, NE Zamboanga). The Cretaceous period, as suggested by the presence of several emplaced Cretaceous ophiolites was characterized by a dominant process of oceanic crust formation, followed by basin closure and lithospheric emplacement. The higher arc magmatic addition rates after the Cretaceous are attributed to the successive subduction-related magmatic episodes, which led to thickened crust in Central Luzon and the Bicol-Negros-Northeastern Mindanao area. The temporal and spatial variations noted in the ophiolite accretion rates could be a function of the active margin-related tectonic processes that took place along the western side of the Philippine archipelago after the Cretaceous period.