Petrogenesis of ultramafic-mafic clasts in the Dos Hermanos Mélange, Ilocos Norte: Insights to the evolution of western Luzon, Philippines

Abstract The clasts of ophiolitic melanges formed in orogenic margins reflect the tectonomagmatic history of a region and record the petrological signatures of oceanic lithospheres that interacted in the past. Exposed at the northwestern edge of Luzon, Philippines, the highly deformed Dos Hermanos Melange (DHM) provides new insights on the complex history of western Luzon island. The DHM is a tectonic melange composed predominantly of ultramafic-mafic clasts set in a sheared serpentinite matrix. The ultramafic clasts are mostly harzburgites with rare occurrences of lherzolite, dunite and chromitite. Petrographic (e.g. protogranular to equigranular texture) and geochemical characteristics (e.g. spinel Cr# = 0.17–0.60, olivine Fo content = 87–91) of the peridotites typify residual mantle peridotites which underwent low to moderately high degrees of partial melting. Mineral chemistry of some dunite and harzburgite samples (e.g. high spinel TiO2 = 0.01–0.64 wt%) further record subsequent modification of the depleted mantle material by arc-related processes (e.g. metasomatism). Most of the mafic clasts classify as gabbros and are composed of highly anorthitic plagioclase (An88–99) and Ti-poor pyroxenes which suggest derivation from arc-related melts. One troctolite clast, however, records the distinct petrographic (e.g. ophitic texture) and geochemical (e.g. low An content of plagioclase = 73–80) signatures of primitive MOR-related magma. These contrasting petrologic signatures in the ultramafic-mafic clasts of the DHM are similar to those observed in the crustal and mantle sections of the Eocene Zambales Ophiolite Complex (ZOC). This suggests that the DHM, like the ZOC, records the complex history of the convergence and emplacement of an ancient oceanic crust onto the Philippine Mobile Belt. Later tectonic processes in the region, which occurred after the emplacement of the ZOC, resulted to the extensive dissection and translation of ophiolitic blocks northwards transforming them into the DHM.