長崎県・上五島の火山岩類の化学的性質

抄録

The Kami-goto district is located at the northern end of the Gotd islands, Nagasaki Prefecture, Southwest Japan, and occupies southwestern margin of the so-called Circum-Japan Sea alkali rock province. The district is devided in two; northern Uku district and southern Ojika district. In Uku district, andesites are mainly distributed, while basalts mainly develop in Ojika district. Generally speaking, the former belong to earlier eruption than the latter, both activities being in Quaternary age. Reaction phenomena between olivine and clinopyroxene have been observed in some but not all of the volcanic rocks which belong to the early stage fractionation, while it can be detected in all of the rocks of the middle stage fractionation. At the same time, large amounts of magnetite crystallize in the latter rocks. The volcanic rocks in Uku district show lower solidification index than 21, while those in Ojika district show higher index, and their chemical characteristics and fractionation trends are shown in Figs. 3 and 6. It can be seen in these figures that these rocks are plotted in a series which has similar tendency as the pigeonitic rock series of KUNO's classification but is situated nearer to Na + K, and represents the fractionation trend of iron enrichment. This kind of trend has never been found in alkali rock series of Japan and surrounding areas. An Extrapolated parental magma is shown in Table 5. This magma has subalkalic characteristic and contains higher amount of Na than K, and is considered to have oceanic characteristic rather than continental. From these results, it is considered that the volcanic rocks in the Kami-goto district were formed by fractional crystallization of subalkalic basalt magma. It is proposed to call these volcanic rocks "Kami-goto rock series".