Old continental zircons from a young oceanic arc, eastern Taiwan: Implications for Luzon subduction initiation and Asian accretionary orogeny

Accretionary orogeny that involves terrain accretion and subsequent reprocessing plays a crucial role in the Phanerozoic tectonics and continental growth of Asia. This study reports zircon U-Pb and Hf isotope data from the Chimei igneous complex, eastern Taiwan, part of the intra-oceanic Luzon arc that has accreted onto the Eurasian continental margin since ca. 5 Ma. Among 292 U-Pb dates and 267 Hf isotope ratios obtained for zircon separates from six andesites, ten grains of magmatic zircons gave a mean Pb-206/U-238 age at 9.0 +/- 0.3 Ma, with epsilon(Hf(t)) values from +20 to +12, which we interpret as the emplacement age of the Chimei complex. Remaining zircons, however, show inherited ages clustering at ca. 14 Ma (n = 9) and ca. 220 Ma (n = 56, the largest peak), along with much older ages of ca. 0.7 Ga, 1.9 Ga, and 2.5 Ga. Whereas the ca. 14 Ma zircons may have crystallized from the earliest magmatism of the northern Luzon arc, the Indosinian and older zircons suggest Cathaysia-type sources that we attribute to a continental fragment that split off from the Eurasian margin by opening of the South China Sea and then drifted and accreted to the western Philippine Sea plate before the Luzon subduction initiation. Consequently, magmas derived from the depleted mantle wedge evolved and picked up the continental zircons during ascent. Our study not only better integrates regional tectonics with magmatic records in Southeast Asia, but also signifies a modern example from Taiwan that highlights the importance of ribbon continents in Asian orogenesis over time and space.