Magmatic response to abrupt changes in geodynamic settings: Pliocene—Quaternary calc-alkaline and Nb-enriched lavas from Mindanao (Philippines)

Abstract

Mindanao, the largest island in the southern Philippine archipelago, is a composite of at least two terranes; one with Eurasian affinity (western Mindanao) and the other belonging to the Philippine Mobile Belt (eastern Mindanao), of Philippine Sea plate affinity. The island is surrounded by three subduction zones that have been installed only in the past 4 m.y. Prior to this, the two terranes were separated by an ocean that disappeared continuously by subduction of its two edges beneath western and eastern Mindanao, where mostly typical arc magmatic rocks, dated at 30 Ma, 19−15 Ma, 12−11 Ma and 7−4 Ma were emplaced. The suturing of the two terranes occurred at ca. 5 Ma. Following this major structural reorganization, abrupt changes are recorded in the < 3 Ma old magmatism of the island. The emplacement of adakites, dated from 1.80 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 western Mindanao, associated Nb-enriched basalts (NEB) and basaltic andesites (2.58−1.08 Ma) might represent either melts of mantle having interacted with adakitic liquids or of heterogenous mantle containing an OIB 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-date potassic volcanism dated at 0.80−0.36 Ma. Among the calc-alkaline rocks, 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 attributed to: 1. (1) the highly heterogenous character of their mantle source, which contains variable amounts of metasomatic pargasite and phlogopite, and, possibly, an additional OIB component that could contribute to Nb enrichment of NEB; 2. (2) the contribution of melts from the ubducted oceanic crust; these melts are either emplaced directly on the surface (adakites) or act as metasomatic agents leading to a Nb-enriched mantle, a probable source of NEB. Garnet and amphibole fractionation could also account for additional variations in the MREE and the HREE.