Amphiboles in the Paleogene Namariyama Granophyres, Eastern San'in District, Southwest Japan

SAKIYAMA Tohru

Journal of science of the Hiroshima University. Series C, Geology and mineralogy

In the eastern San'in district, a volcano-plutonic complex of the late Paleogene age is widely developed. The Namariyama granophyres belonging to the Namariyama intrusive rocks are one of the main constituting members of the complex. Judging from the mode of occurrence and textures, amphiboles in the granophyres are inferred to be classified into the following three groups, that is, amphiboles -I, II and III in the order of crystallization. Amphibole-I is scarsely zoned, and is titaniferous ferroan pargasite to titaniferous ferroan pargasitic honblende in composition. Amphibole-II is mainly composed of magnesio hornblende. It shows the zonal structure represented by increase of Si from core to rim in each grain. It is normally zoned in Si-poor crystal and reversely zoned in Si-rich one with respect to the mg-value. Amphibole-III is actinolitic hornblende to actinolite. It has zonal structure and Si and mg-value increases from core to rim.
In the progress of the crystallization of amphiboles, Si increases and Al, Ti, Na and K decrease. On the other hand, mg-value decreases in the early stage and increases in the late stage during the crystallization of the Namariyama granophyres.
Si enrichment in the later stage of crystallization can be explained by increase of silica activity in the liquid with crystallization. Increase of oxygen fugacity in the later stage of crystallization has resulted in the appearence of magnetite and the increase of mg-value of amphibole.
Ti/A1 ratio of amphibole in the Namariyama granophyres is the highest among those in the late Cretaceous to Paleogene plutonic rocks in the Innerside of Southwest Japan. This fact indicates that amphibole in the Namariyama granophyres has been crystallized under the condition of comparatively high temperature at shallow level.
Appearence of Ti-pargasite supports the activity of basic magma has been closely connected with the genesis of the Namariyama granophyres.

英語

HIROSHIMA UNIVERSITY

[ISSN] 0075-4374

[NCID] AA00706718