Volcaniclastic sedimentation associated with trachytic volcanism in an oceanic intraplate volcano (Dokdo volcano, Republic of Korea)

Abstract

Dokdo is an erosional remnant of an oceanic intraplate volcano built on the backarc basin floor off the Korean peninsula. Basaltic and trachytic volcaniclastic rocks named the Dongdo and Seodo tuffs, respectively, are sandwiched between the submarine and subaerial volcanic rocks of the island. The Dongdo Tuff consists of polymictic breccia of variably vesicular and varicolored basalt clasts in a trachytic pumice-rich or a pumice-poor matrix. The tuff represents a volcaniclastic apron deposited by submarine to subaerial debris flows fed from an emergent basaltic volcano. An explosive trachytic eruption produced abundant pumice in the tuff matrix. The eruption was followed by trachytic cryptodome intrusion and explosion, resulting in the deposition of the Seodo Tuff by block-and-ash flows. The block-rich and the lapilli- and ash-rich facies of the deposits were selectively deposited on different parts of the island because of the control of the paleotopography on the flows. The Seodo Tuff is composed mostly of crystalline trachytic rock fragments, alkali feldspar crystals, and rarely of pumiceous particles and glass shards. Some ash particles are aggregated or coated by extremely fine ash. The componentry suggests that the block-and-ash flows originated from explosive pulverization of a significantly degassed, crystallized, and mostly solidified lava dome. Explosive vaporization of external water that permeated the lava dome and magmatic gas expansion in the dome interior is inferred to be responsible for the coarse to fine fragmentation of the lava dome. The Dongdo and Seodo tuffs probably represent tephra-producing and dome-forming phases of a trachytic eruption after the transition from a basaltic shield to a post-shield stage of an oceanic intraplate volcano. They therefore provide a glimpse into the relatively under-studied trachytic volcanism associated with the post-shield stage of oceanic intraplate volcanoes.