HIGH-FIELD STRENGTH ELEMENT ENRICHMENT OF PLIOCENE-PLEISTOCENE ISLAND-ARC BASALTS, ZAMBOANGA PENINSULA, WESTERN MINDANAO (PHILIPPINES)

The Pliocene-Pleistocene magmatic activity of the Zamboanga are is lin ked to the southward subduction of the Oligocene-Miocene Sub Sea back- are basin along the Sulu Trench. The magmatic products include small a mounts of adakites dated from 3.8 to 0.7 Ma, abundant Nb-enriched basa lts 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 NE B are distinguished: nearly primitive Mg-rich (MG) basalts displaying positive or no Nb anomalies with respect to adjacent incompatible elem ents and more evolved low-K (LK) and calc-alkaline (CA) lavas which, d espite their Nb enrichment, display negative Nb anomalies. Although th e 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 m ight be present in their source. We suggest that these minerals could be added metasomatically to the mantle through hybridization by percol ating slab melts, during which Nb and Ti are preferentially extracted from the adakitic melts. In an early stage (4-3 Ma) of the subduction oftheyoung and hot Sub Sea basin crust beneath the Zamboanga peninsula , adakitic liquids formed at depths of 75-85 km. A few of them were em placed at the surface but most were consumed through slab melt-mantle metasomatic reactions Adakite production and emplacement continued lat er (<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.