Regional geology of the Bangui Formation: insights offered by sedimentary geochemistry into the early evolution of an island arc

Abstract The Paleogene magmatic arc in Luzon marks the onset of island arc formation within the Philippine island arc system. This study examines the Late Eocene to Late Oligocene Bangui Formation, the oldest sedimentary sequence exposed in Northern Luzon. Whole-rock geochemical compositions of its clastic rocks are presented for the first time to provide key insights into its provenance and tectonic setting. Bivariate and ternary diagrams, using major (e.g., TiO 2 vs Al 2 O 3 ) and trace element compositions (e.g., low La/Th and low Hf), suggest multiple sources for the Bangui Formation. Tectonic discrimination diagrams, showing low La (< 20) and Th (< 5) concentrations, indicate an oceanic island arc setting. The mineral chemistry of the Bangui sandstones was also analyzed for the first time. The results reveal that the plagioclases contain low orthoclase content, with some grains exhibiting a more albitic composition. Whole-rock geochemistry and mineral chemistry data suggest that the clasts of the Bangui Formation were derived from the volcanic unit of the Caraballo Formation, with contributions from intermediate to felsic units in the Caraballo Range. It is inferred that an Early Eocene (or older) proto-North Luzon Arc source (pNLA) contributed to the Bangui clastic rocks. Our results show that the integrated use of whole-rock geochemical and mineral chemistry data, complementary to field geological data, offers a robust approach to deciphering the provenance and tectonic setting of clastic rocks in northern Luzon. The field geological and geochemical signatures of these clastic rocks provide crucial information on the early Cenozoic history of the Philippine island arc system. This hopefully can contribute to unravelling the complex geologic history of this part of the Western Pacific region.