The Role of Crustal Thickness in the Formation of Adakitic Rocks in an Island Arc Setting:  Insights from the Philippines

Carla B. Dimalanta¹ and Graciano P. Yumul, Jr.^1,2

¹ National Institute of Geological Sciences, College of Science,
University of the Philippines, Diliman, Quezon City, Philippines 1101
² Philippine Council for Industry and Energy Research and Development,
Department of Science and Technology, Bicutan, Taguig, Philippines

Abstract

A geochemically distinct class of rocks subduction-related volcanic rocks known as adakites has been recognized recently. A large number of studies had been carried out to look into the manner by which this type of rocks is generated. Adakites were initially thought to form from the partial melting of subducted young, hot oceanic slabs. Recent investigations, however, reveal a variety of mechanisms that lead to the formation of adakites. In addition to the subduction of young oceanic crusts, adakites may also be produced by magmatism resulting from oblique subduction, low-angle or flat subduction, as well as the initiation of subduction or slab-tearing. In investigations of adakites, some workers pointed out that the partial melting of thickened crust can explain the generation of adakitic melts. How thick should the crust be in order to form adakites? Defant and Drummond (1990) suggested that slab melting occurs within the amphibolite-eclogite transition zone that corresponds to a 70-80 kilometer depth range.

Estimates of crustal thickness in the Philippines indicate that the archipelago is made up of crust with a thickness ranging from ~17 to 30 kilometers. However, there are portions of the archipelago which are characterized by thickened crust (>30 kilometers). Looking at the distribution of adakites and crustal thickness, one sees no distinct correlation between the occurrence of adakites and crustal thickness. Adakites are found in areas underlain by normal crustal thickness as well as in areas underlain by thickened crust. However, in the La/Sm versus Sm/Yb diagram, adakites within the thickened portions of the archipelago plot mostly within the amphibole field. This suggests that, in the case of Philippine adakites, amphibole plays a significant role in the generation of adakitic magmas.