Zircon U-Pb and Hf isotopic constraints on the magmatic evolution of the Northern Luzon Arc

Abstract

The complete volcanic sequences restored in the Coastal Range of Taiwan are key archives for better understanding the magmatic and tectonic evolution of the Northern Luzon Arc. This paper reports (1) new zircon U-Pb ages and Hf isotopic data of fourteen volcanic samples from different sequences of four major volcanoes in the Coastal Range, (2) Hf isotopic data of dated magmatic and detrital zircons from two offshore volcanic islands, Lutao and Lanyu. These data indicate that the arc magmatism in the Coastal Range started at ∼15 Ma, most active at ∼9 Ma, and ceased at ∼4.2 Ma. Magmatic zircons from the arc rocks show a significant variation in Hf isotopic composition, with ϵHf(T) values varying from +24.9 to +4.8. As pointed out by our previous studies, old continental zircons that show Cathaysian-type ages and Hf isotope features are common in samples from the Yuemei, Chimei, and Lanyu volcanoes, supporting the notion for the influence of the existence of an accreted micro-continent or continental fragment plays a role in the petrogenesis. Such inherited zircons are not observed in the Chengkuang'ao and Tuluanshan volcanoes and uncommon in Lutao, implying the discontinuity or a limited extent of the accreted continental fragment. The ϵHf(T) values are high and positive from ∼15 - 8 Ma (+25 to +15; ±5ϵ-unit variation), and became lower from ∼6 to 4.2 Ma (+20 to +8; ±6ϵ units) and the lowest from ∼1.3 Ma (+19 to +5; ±7ϵ units). Such a temporal variation in zircon Hf isotopic ratios can be also identified in whole-rock Hf and Nd isotopic compositions, which decrease from ∼6 Ma when the Northern Luzon Arc may have started colliding with the Eurasian continental margin.