Zircon U-Pb Age Determination of Volcanic Eruptions in Lutao and Lanyu in the Northern Luzon Magmatic Arc

This paper reports for the first time zircon U-Pb ages of volcanic rocks and sands from Lutao and Lanyu, two islets off SE Taiwan in the north Luzon arc. The samples include (1) seven andesites from four volcanic units and three river/beach sands from Lutao and (2) five basaltic andesites from four volcanic units and two river/beach sands from Lanyu. The Lutao andesites contain abundant magmatic zircons, aging from ~1.54 to ~1.24 Ma for individual sample, which yielded an overall mean 206Pb/238U age of 1.31 ± 0.03 Ma (n = 190, MSWD = 2.6). This is slightly older than, or broadly coincident with, a mean 206Pb/238U age of 1.23 ± 0.03 Ma (n = 103, MSWD = 1.9) given by detrital zircons from the three sands. The Lanyu volcanics appear to have less abundant magmatic zircons, aging from ~2.72 to ~2.35 Ma for individual sample, which yielded an overall mean 206Pb/238U age of 2.61 ± 0.13 Ma (n = 11, MSWD = 1.8). This accords with a mean 206Pb/238U age of 2.69 ± 0.11 Ma (n = 34, MSWD = 4.7) obtained by detrital zircons from the two sands. The age data suggest that in Lutao and Lanyu the major volcanic eruptions occurred at ~1.3 and ~2.6 Ma, respectively. Moreover, volcanic samples from both islets contain various amounts of older inherited zircons, ~11% in Lutao and up to ~82% in Lanyu, which together with detrital zircons from the sands show main age peaks at ~150 Ma and ~1.9 and ~2.5 Ga, consistent with the notion for a "hidden" continental crust involved in the genesis of the northern Luzon magmatic arc.