MAGMATIC RESPONSE TO ABRUPT CHANGES IN GEODYNAMIC SETTINGS - PLIOCENEQUATERNARY CALC-ALKALINE AND NB-ENRICHED LAVAS FROM MINDANAO (PHILIPPINES)

Mindanao, the largest island in the southern Philippine archipelago, i s a composite of at least two terranes; one with Eurasian affinity (we stern Mindanao) and the other belonging to the Philippine Mobile Belt (eastern Mindanao), of Philippine Sea plate affinity. The island is su rrounded by three subduction zones that have been installed only in th e past 4 m.y. Prior to this, the two terranes were separated by an oce an that disappeared continuously by subduction of its two edges beneat h western and eastern Mindanao, where mostly typical arc magmatic rock s, dated at 30 Ma, 19-15 Ma, 12-11 Ma and 7-4 Ma were emplaced. The su turing of the two terranes occurred at ca. 5 Ma. Following this major structural reorganization, abrupt changes are recorded in the <3 Ma ol d magmatism of the island. The emplacement of adakites, dated from 1.8 0 to 0.10 Ma and 0.70 Ma in eastern and western Mindanao, respectively , is thought to result from the initiation of new subductions. In west ern Mindanao, associated Nb-enriched basalts (NEB) and basaltic andesi tes (2.58-1.08 Ma) might represent either melts of mantle having inter acted with adakitic liquids or of heterogenous mantle containing an OI B component. Seismic data give evidence of the presence of a detached subducted slab beneath Central Mindanao, where the recent volcanism is post-collisional. Calc-alkaline lavas, dated at 2.31-0.29 Ma, pre-dat e potassic volcanism dated at 0.80-0.36 Ma. Among the calc-alkaline ro cks, a group of relatively magnesian basalts and basaltic andesites is also considered to be enriched in Nb and one adakitic sample has been found. The geochemical diversity of magmatic types in Mindanao is att ributed to: (1) the highly heterogenous character of their mantle sour ce, which contains variable amounts of metasomatic pargasite and phlog opite, and, possibly, an additional OIB component that could contribut e to Nb enrichment of NEB; (2) the contribution of melts from the subd ucted oceanic crust; these melts are either emplaced directly on the s urface (adakites) or act as metasomatic agents leading to a Nb-enriche d mantle, a probable source of NEB. Garnet and amphibole fractionation could also account for additional variations in the MREE and the HREE .