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.
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