The volcanic history of the late Pleistocene Shamao complex, Tatun Volcano Group, northern Taiwan

Although the Tatun Volcanic Group (TVG) looms over the metropolitan capital region of Taiwan, the threats it poses are only vaguely known. Here, we investigate the Shamao complex, one of the youngest features of the TVG. Analysis of a high-resolution, LiDAR-based, Digital Terrain Model (DTM), augmented by outcrops and a drill core, reveals that the complex consists of overlapping West and East Domes and an older West Flow, which extends southwestward ~ 650 m from a source on the flank of the domes. High precision ⁴⁰ Ar/ ³⁹ Ar analyses reveal that West Flow erupted at 196.4 ± 2.3 ka, long before the eruption of East and West Domes at 24.2 ± 1.8 ka. The three principal features of the complex consist of crystal-rich volcanic rocks of basaltic andesitic to andesitic bulk compositions. However, they display petrographic and geochemical differences, most likely due to differing mineralogy and melt fraction during extraction from hydrous mafic sources in the crust. Overall, we suggest that a magmatic system involving mafic composition with elevated magmatic water content, intermittent magma extraction with cumulate formation over a 10–100 kyr timescale and remobilization of accumulative crystals by later magma pulses, explain all essential geomorphologic, ⁴⁰ Ar/ ³⁹ Ar geochronologic, petrologic, and geochemical features of the Shamao complex.