Abstract
In the Nakanomata prospecting area, Yamagata Prefecture, an igneous complex intrudes into the strata that are composed of the pyroclastic rocks and the lava flows in the lower Miocene.
The pyroclastic rocks are divided into two parts; andesitic lapilli tuff_??_tuff breccia at the lower part and dacitic lapilli tuff_??_tuff breccia or volcanic breccia at the upper part. The upper part is characterized by welded tuff. The pyroclastic rocks might accumlate on land as the pyroclastic flow.
The main rock facies of the igneous complex may be named as gabbro, diorite, granodiorite, andesite and dacite.
Fissure patterns are vertical or parallel to the general trend of the geological structure in this area, and the veins which have been prospected occure filling the former. Ore minerals are pyrite, sphalerite, galena and chalcopyrite, and gangue minerals are quartz, calcite and chlorite.
In this paper, the problem if the so-called Tertiary granitic masses including this complex in northeastern Japan are products as the plutonism or as the volcanism is proposed.
