Co-located monogenetic eruptions similar to 200 kyr apart driven by tapping vertically separated mantle source regions, Chagwido, Jeju Island, Republic of Korea

Abstract

New eruptions in monogenetic volcanic fields conceptually occur independently of previous ones. In some instances, however, younger volcanic structures and vents may overlap with older edifices. The genetic links between such co-located eruptions remain unclear. We mapped and analysed the stratigraphic relationships between eruptive units on the 400x900-m island of Chagwido off the western coast of Jeju Island, a Pleistocene to Holocene intraplate volcanic field. Chagwido consists of an eastern, older tuff ring with a nested scoria cone and a western tuff, scoria and lava flow sequence. The two stratigraphic packages are separated by a prominent paleosol. The East-Chagwido tuff and scoria deposits were eroded and a period of intense weathering and soil development occurred, before a subsequent West-Chagwido tuff ring and scoria cone and lava complex was erupted. The two eruptions were fed by three chemically distinct magmas. The older eastern eruption consists of magma with composition transitional between high-Al alkalic basalt and low-Al alkalic basalt and has stratigraphic characteristics, composition and syneruptive trends akin to the neighbouring Dangsanbong tuff cone. This magma type is typical for the transitional stage from high-Al alkalic (pre 500 ka) to low-Al alkalic (post 250 ka) identified for the greater Jeju volcanic system. The East-Chagwido volcanic complex thus formed as the western-most in a chain of three volcanoes along a fissure system, with a small volcanic remnant island Wado 1 km to the east and the large Dangsanbong tuff cone another 1 km eastward. A new Ar/Ar age of 446 +/- 22 ka for Dangsanbong likely characterizes the age of the whole chain. The second, West-Chagwido eruption started with low-Al alkalic basalt forming a phreatomagmatic phase and ended with subalkalic basalt forming a scoria cone and lava flows. The occurrence of subalkalic lavas is known across Jeju to have started only at similar to 250 ka, and thus, the well-developed paleosol represents at least 200 kyr between the two co-located eruptions. The distinctive magma compositions show that each eruption tapped an independent region within the same underlying mantle source. These observations show that contrary to most assumptions of monogenetic volcanism, an already "tapped" source region may become fertile again through mantle convection/migration and eruptions can thus be expected from old vent sites in long-lived volcanic fields.