An Alkaline Rock Body in Tanegashima Island, South Kyushu, Japan

Abstract

An alkaline intrusive rock body with peculiar occurrence and petrographic character is found in the Tanegashima island, south Kyushu, Japan. It is exposed as an extraordinary thin dyke intruded into the Paleogene Kumage group, and overlain by the Pliocene-Pleistocene Kaminaka group. It is only about 10m or less in thickness but as long as 20km or more in length, in a NNE-SSW direction which may be concerned with the “Peri-Tunghai tectonic movement”. It is composed of albite-oligoclase, anorthoclase, “kaersutite”, biotite, opaque minerals and some accessories such as chlorite, calcite, prehnite, pumpellyite, etc. The colour index is 40～50. The intrusive rock with such a mineral composition may be called vogesitic kersantite (somewhat leucocratic variety) belonging to the lamprophyres. It is to be noted that the K-Ar age of this dyke rock is 16±2 m.y., and the magnetic polarity is Re.