Geochronological and geochemical constraints on juvenile continental crust formation in the Eocene Luzon intra-oceanic arc, Philippines

Abstract The formation of juvenile felsic crust in intra-oceanic arcs (IOAs) represents a fundamental but poorly understood process in Earth's crustal evolution. While Archean tonalite-trondhjemite-granodiorite (TTG) suites have been extensively studied, the mechanisms driving post-Archean crustal growth in IOAs, particularly in the absence of pre-existing continental material, remain unclear. The Luzon Granitoid Complex (LGC) in the Central Cordillera of Northern Luzon, Philippines, offers valuable insights into juvenile crustal growth within a late Eocene (34–36 Ma) intra-oceanic arc system. Integrated analysis of U–Pb zircon geochronology, Hf isotopes, whole-rock geochemistry, and petrography classifies the LGC as low-pressure intra-oceanic arc granitoids (LP-IOAGs), comprising tonalites and trondhjemites with calc-alkaline compositions and primitive oceanic arc signatures. These LP-IOAGs exhibit diagnostic low-pressure characteristics (Sr/Y < 20, La/Yb < 10, flat HREEs), indicating plagioclase-dominated fractionation in a relatively thin arc crust without garnet involvement. Our results reveal two distinct petrogenetic signatures: (1) partial melting of gabbroic-amphibolitic lower crust (LREE-enriched Group 2: LaN/YbN > 2) and (2) fractional crystallization of mafic magmas (flat REE Group 1: LaN/YbN < 2). This heterogeneity in formation mechanisms, occurring simultaneously within the same arc segment, challenges conventional evolution models that ascribe specific processes to discrete spatial or temporal stages. Furthermore, zircons from the LGC exhibit consistently depleted mantle-like εHf(t) values (+ 13 to + 15), confirming derivation from purely juvenile sources without crustal recycling. These results show that low-pressure intra-oceanic arc granitoids (LP-IOAGs) can produce continental crust-like signatures via chemically distinct, yet coeval magmatic processes, independent of ancient crustal contributions. This advances our understanding of post-Archean crustal growth mechanisms.