Age, geochemical and isotopic variations in volcanic rocks from the Coastal Range of Taiwan: Implications for magma generation in the Northern Luzon Arc

This paper reports the first systematic analysis of age and geochemical variations in volcanic rocks from the Coastal Range of Taiwan, the Northern Luzon Arc. The rocks, recovered from four main volcanoes, vary from low-K tholeiitic to medium-K calc-alkaline basalts to dacites. The rocks are typical of arc magmatic products, exhibiting enrichment in the large ion lithophile elements and depletion in the high field strength elements. Our new ⁴⁰Ar/³⁹Ar age data constrain the youngest eruption time in each of the four volcanoes, i.e., from north to south, at ~ 7.2 Ma (Yuemei), ~ 4.2 Ma (Chimei), ~ 6.2 Ma (Chengkuang'ao) and ~ 8.5 Ma (Tuluanshan), respectively. These data indicate that volcanism in the Northern Luzon Arc did not cease progressively from north to south, as previously alleged. The high and broadly uniform Nd isotope ratios [ε_Nd = + 10.1 to + 8.8] and trace element characteristics of the rocks suggest a principal magma source from the depleted mantle wedge. Their overall geochemical variations are ascribed to magma chamber processes. The effects of magmatic differentiation and crustal contamination differ among each volcano, most likely owing to the discrepancy of residence time in individual magma chambers. Consequently, we propose a binary mixing model for the magma generation that involves arc magmas sourced from the depleted mantle wedge and up to ~ 5% crustal contamination with a continental fragment split off from the Eurasian margin.