Fission track ages of volcanic rocks from the Hikosan area, northern Kyushu ( Japan).

Fission track ages have been determined on zircon grains separated from Neogene volcanic rocks in the Hikosan area to clarify their chronological relations to the 'Green Tuff' activity. The obtained ages of 10 rock samples and their standard errors are as follows in ascending order: Yamakuni Formation 5.68+ or -0.29 Ma, 4.50+ or -0.48 Ma, Kitasakamoto Formation 5.66+ or -0.30 Ma, 4.83+ or -0.51 Ma, 5.22+ or -0.56 Ma, 4.60+ or -0.35 Ma, Hikosan Volcanic Rocks 4.05+ or -0.78 Ma, 3.97+ or -0.39 Ma, 3.95+ or -0.40 Ma, 3.85+ or -0.40 Ma. The 2 age data for the Yamakuni Formation seem to be lowered due to alteration, while those for the Kitasakamoto Formation are likely to present their formation age. Therefore, the so-called 'Green Tuff' activity in N and central Kyushu is considered to have continued on into the Pliocene. On the other hand, the Hikosan Volcanic Rocks were formed in the Pliocene, contrary to a previously accepted view that they are assigned to the Pleistocene Hohi Volcanic Rocks. -from English summary