Ages and Geochemistry of Igneous Rocks in the Northern Sierra Madre-Caraballo Mountains, Philippines: Implications for Subduction Reconstruction Beneath Northern Luzon

Subduction tectonics related to Late Paleogene island-arc magmatism in Northern Luzon, the Philippines, has been debated for some time. New field data, combined with zircon U–Pb dating and Hf isotopic studies, whole-rock element and Nd isotopic data from the Caraballo Formation in the Northern Sierra Madre reveal two suites of island-arc magmatic rocks: (1) ~34 Ma igneous rocks characterized by low-K₂O (≤ 0.6 wt.%), low Nb/Yb (0.11–0.26), and high ¹⁴³Nd/¹⁴⁴Nd (εNd_[t] = 9.5–9.9), and (2) ~31 Ma igneous rocks exhibiting progressive enrichments of K₂O (~0.8 wt.%), Nb/Yb (0.37–0.76), and ¹⁴³Nd/¹⁴⁴Nd (εNd_[t] = 8.2–9.1). These differences reflect a shift in subduction influences, transitioning from slab fluids of an old subducted slab to slab melts with adakitic characteristics of young subducted slab between 34 and 31 Ma. The ~46 Ma Dupax Batholith in the Caraballo Mountains shows similar geochemical and isotopic signatures as the ~34 Ma igneous rocks of the Caraballo Formation (K₂O: ~0.7 wt.% vs. ≤ 0.6 wt.%; Nb/Yb: 0.24 vs. 0.11–0.26; zircon εHf_[t]: 13.8–15.8 vs. 13.7–15.7). This suggests a similar mantle source and subduction influx in both cases. When considered in conjunction with available geochronological and geochemical data from the Northern Sierra Madre, the Caraballo Mountains, and the Central Cordillera, this suggests that arc growth of Northern Luzon may have started as early as the Middle Eocene. The eastern and western ranges of Northern Luzon comprised the Proto-Luzon Arc before the intra-arc rifting started in the Late Oligocene–Early Miocene. Unlike a Neogene cessation of island-arc magmatism in the Northern Sierra Madre, there was a persistent arc growth in the Central Cordillera. It suggests that the Central Cordillera always situated in the frontal arc while the Northern Sierra Madre evolved from a Late Paleogene back arc to a Neogene remnant arc. This supports a continuously east-dipping subduction along the western plate boundary of Northern Luzon. We conclude that the change of subduction influence in the Northern Luzon Arc during the Middle Eocene to Early Oligocene is linked to the migration of the subducted slab from the Mesozoic Eastern Neo-Tethys to the Eocene Celebes at ~31 Ma.