Zircon LA-ICPMS U–Pb ages and Hf isotopes of Huayu (Penghu Islands) volcanics in the Taiwan Strait and tectonic implication

Huayu, the only non-basalt islet in the Penghu Islands, consists mainly of andesite lavas intruded by some intermediate (dacite/granodiorite) and felsic (rhyolite/granite) dikes. The LA-ICPMS U–Pb zircon age determination yields 437 ± 13 Ma (n = 16, MSWD = 6.9) for the andesite lava and 63.3 ± 1.5 Ma (n = 7; MSWD = 2.9) for its potassic altered zone, and 58.7 ± 0.8 Ma (n = 14; MSWD = 2.5) for one rhyolite dike. The old and young groups of zircon show distinctive Hf isotope compositions: εHf(T) = −9 for ∼437 Ma zircons vs. εHf(T) = +9 to +6 and +9 to −1 for ∼63 Ma and ∼59 Ma zircons. On the other hand, Sr and Nd isotopic compositions of the andesite lava (ISr = 0.7034 and εNd(T) = +3.9 if T = 437 Ma) are more depleted than the alteration zone and rhyolite dike (ISr = 0.7051 and εNd(T) = −1.3 to −1.9). Decoupling of zircon εHf(T) and whole-rock εNd(T) values of the andesite lava rules out the possibility of 437 ± 13 Ma to be the eruption time. Alternatively, based on the similarity between the published zircon fission track ages (65 ± 3 Ma for the andesite lava and 61 ± 2 Ma for the rhyolite dike; [Yang, H.C., Chen, W.S., Huang, J.C., Wang Lee, C.M., 2008. Volcanic lithofacies of the Huayu Island. Bulletin of Central Geological Survey 21, 143–164 (in Chinese with English Abstract)]) and our U–Pb zircon ages of the altered andesite zone and rhyolite dike, the span of 65–63 Ma is suggested to be the formation time of Huayu andesites. Since Huayu is situated in a NE–SW trending high-magnetic anomaly belt which has been generally interpreted as a remnant magmatic terrain, these data, along with the geochemical results, provide insights of this belt as a split part of the Southeast Coast Magmatic Belt in which Huayu volcanics represent the younger arc volcanism comparable with those appeared in southern Fujian.