Magmatic source enrichment by slab-derived melts in a young post-collisionsetting, central Mindanao (Philippines)

Central Mindanao was the locus of a Pliocene (4-5 Ma old) are-are collision event followed by basaltic to dacitic magmatism starting at 2.3 hla, repre senting the most voluminous volcanic field in the Philippines. Lava composi tions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched bas alts are among the magmatic products. All the lavas are Na-rich (up to 4.88 %), with Na2O/K2O ratios from 2.5 to 6.5. Sr, Nd and Pb isotopic compositio ns are similar to MORB, except for some shoshonitic lavas that have slightl y less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melti ng of an enriched source. Trace element systematics indicate that the sub-c entral Mindanao mantle is characterized by the presence of earner, phlogopi te, amphibole, and perhaps some titanate phase. The enrichment of this sour er is attributed to the interaction of slab-derived melts, i.e., adakites, with the are mantle. This would explain the presence of Nb-enriched basalts , transitional adakites and high-magnesium andesites, as well as the bulk N a-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, L ILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions. The MORB-like isotopic signatures of the central Mindanao la vas preclude important contributions of slab-derived hydrous fluids, sedime nts, continental crust or an GIB-type contaminant. Slab melting after cessa tion of subduction is deemed possible by thermal rebound of previously depr essed geotherms, Initial contributions to mantle enrichment in post-collisi on sites may thus come from slab melts. In most other cases of post-collisi on magmatism, however, this signature can be easily masked by enrichments c oming from other sources, e.g.. the continental lithosphere. (C) 2000 Elsev ier Science B.V. All rights reserved.