High field strength element enrichment of Pliocene-Pleistocene island arc basalts, Zamboanga Peninsula, Western Mindanao (Philippines)

Abstract

The Pliocene-Pleistocene magmatic activity of the Zamboanga arc is linked to the southward subduction of the Oligocene-Miocene Sulu Sea back-arc basin along the Sulu Trench. The magmatic products include small amounts of adakites dated from 3·8 to 0·7 Ma, abundant Nb-enriched basalts and basaltic andesites (NEB) dated from 2 to 1 Ma and a lone calc-alkaline potassic basaltic andesite dated at 0·4 Ma. Three kinds of NEB are distinguished: nearly primitive Mg-rich (MG) basalts displaying positive or no Nb anomalies with respect to adjacent incompatible elements and more evolved low-K (LK) and calcalkaline (CA) lavas which, despite their Nb enrichment, display negative Nb anomalies. Although the role of OIB-type mantle components has been advocated to explain the HFSE enrichment of NEB, the spatial and temporal association of these rocks with adakites suggests a petrogenetic link between them. Trace element characteristics of the NEB imply that amphibole and ilmenite might be present in their source. We suggest that these minerals could be added metasomatically to the mantle through hybridization by percolating slab melts, during which Nb and Ti are preferentially extracted from the adakitic melts. In an early stage (4-3 Ma) of the subduction of the young and hot Sulu Sea basin crust beneath the Zamboanga peninsula, adakitic liquids formed at depths of 75-85 km. A few of them were emplaced at the surface but most were consumed through slab melt-mantle metasomatic reactions. Adakite production and emplacement continued later (<2 Ma), while the Nb-enriched mantle was brought by convection to depths that allowed its melting and the subsequent emplacement of NEB behind the adakitic front of the Zamboanga arc.