Issue 3: 1996

Abstract

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

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

    • FG. Sajona, RC. Maury1, H. Bellon1, J. Cotten1 and M. Defant2 1URA-CNRS 1278 et Universite de Bretagne Occidentale, 6, Avenue le Georgeu, BP 809, F-29285 Brest Cedex, France, 2Department of Geology, University of South Florida, Tampa, FL 99690, USA and To whom correspondence should be addressed at: Mines and Geosciences Bureau, North Avenue, Diliman, 1100 Quezon City, Philippines

    ABSTRACT

    The Pliocene-Pleistocene maymatic activity of the Zamboanga arc is linked to the southward subduction of the Oligocene-Miovene Sulu Seu back-arc basin along the Sulu Trench. The magmalic products include small amounts of adakites dated from 3.8 to 0.7 Ma, abundant Nb-enriched basalts and basaltic andersites (NEB) dated from 2 to 1 Ma and a lone calc-alkaline potassic basaltic andosite dated at 0.4 Ma. Three kinds of NEB are distinguished: nearly primative Mg-rich (MG) basalts displaying positive or no Nb anomalies with respect to adjacent incompatible elements and more evolved low-K (LK) and calc-alkaline (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 associated of these rocks with adakites suggests a patrogenetic 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 hydridization 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 Sula Sea basin crust beneath the Zambuanga 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.

    Keywords: adakite; melasomalism; Mindanao; Nb-enriched basalts; subduction

    Pages: 693 - 726

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