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Magmatic source enrichment by slab-derived melts in a young post-collision setting, central Mindanao (Philippines) |
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| Authors: | Fernando G Sajona Ren C Maury Manuel Pubellier Jacques Leterrier Herv Bellon and Joseph Cotten |
| Institution: | a Mines and Geosciences Bureau, North Ave., Diliman, 1100 Quezon City, Philippines b Département des Sciences de la Terre et UMR 6538 Université de Bretagne Occidentale 6, Avenue le Gorgeu, BP 809, 29285 Brest Cedex, France c Laboratoire de Géologie, Ecole Nationale Supérieure, URA 1316, 24 Rue Lhomond, 75231 Paris Cedex 05, France d Centre de Recherches Pétrographiques et Géochimiques, CNRS, UPR 9046, 15, rue Notre Dame des Pauvres, BP 20, 54501 Vandoeuvre Les Nancy Cedex, France |
| Abstract: | Central Mindanao was the locus of a Pliocene (4–5 Ma old) arc–arc collision event followed by basaltic to dacitic magmatism starting at 2.3 Ma, representing the most voluminous volcanic field in the Philippines. Lava compositions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched basalts 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 compositions are similar to MORB, except for some shoshonitic lavas that have slightly less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melting of an enriched source. Trace element systematics indicate that the sub-central Mindanao mantle is characterized by the presence of garnet, phlogopite, amphibole, and perhaps some titanate phase. The enrichment of this source is attributed to the interaction of slab-derived melts, i.e., adakites, with the arc mantle. This would explain the presence of Nb-enriched basalts, transitional adakites and high-magnesium andesites, as well as the bulk Na-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, LILE 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 lavas preclude important contributions of slab-derived hydrous fluids, sediments, continental crust or an OIB-type contaminant. Slab melting after cessation of subduction is deemed possible by thermal rebound of previously depressed geotherms. Initial contributions to mantle enrichment in post-collision sites may thus come from slab melts. In most other cases of post-collision magmatism, however, this signature can be easily masked by enrichments coming from other sources, e.g., the continental lithosphere. |
| Keywords: | Post-collision magmatism Plio-Quaternary volcanism Slab melting Adakites Mantle source enrichment |
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