Abstract
The Chilungshan andesite belongs to the Chilung Volcano Group in northern Taiwan and constitutes the western end of the island chains of the Ryukyu arc. Chemically it is a normal, medium-K calc-alkaline andesite. SHRIMP dating of magmatic zircons yielded an emplacement 206Pb/238U age of 1.04±0.06Ma with εHf(t) of 6.1 to 13.4 and δ18O of 5.02 to 6.28‰. The zircons contain inherited cores and felsic to andesitic silicate-melt (glass) inclusions besides apatite, quartz, K-feldspar, biotite, garnet, Fe-Mg minerals (pyroxene or hornblende) and plagioclase with variable An values. Plagioclase phenocrysts also contain quartz, K-feldspar and felsic melt inclusions. This study confirms the presence of micro-scale heterogeneity in the andesite as a result of mixing between mantle-derived and continental material. Contamination of continental crust occurs either within the magma chamber at a medium to deep crustal level and/or during dehydration and melting of a subducted oceanic slab and overlying sediments in an island-arc environment.
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