Micro-scale heterogeneity of andesite from Chilungshan, northern Taiwan: Evidence from melt inclusions, geochronology and Hf–O isotopes of zircons

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 ± 0.06 Ma 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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► The Chilungshan andesite belongs to the Chilung Volcano Group in northern Taiwan.
► Magmatic zircon of 1.04 Ma contains felsic to andesitic silicate-melt inclusions.
► This study confirms the presence of micro-scale heterogeneity in the andesite .
► It is a result of mixing between mantle-derived and continental material in an island-arc environment.