三郡変成岩中の泥質片岩より産するローソン石

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Lawsonite is a calcium-aluminosilicate characteristically occurs in the high pressure−low temperature type metamorphic terranes. It occurs ubiquitously in pelitic, psammitic, and mafic rocks with sodic amphibole in the typical high P/T type metamorphic terranes such as Franciscan and New Caledonia (MIYASHIRO, 1973 ; TURNER, 1980, etc.). However, in the Sangun metamorphic belt of which mineral assemblages represent lower pressure type of glaucophanitic metamorphism, lawsonite was thought to occur only in a part of mafic rocks and a part of psammitic rocks (SAKAKI & YAMAMOTO, 1967 ; HASHIMOTO & IGI, 1970 ; NISHIMURA & OKAMOTO, 1976).
The occurrence of lawsonite is very rare, so its stability range in the zonation of the Sangun metamorphism is not yet established. The rare occurrence is considered to be delicately controlled by bulk chemical compositions of small domains of centimeters or millimeters scale. According to the detail description by Nishimura & Okamoto (1976), lawsonite has close relation with chlorite-albite assemblage.
In quartzofeldspathic schists, Iawsonite was not recognized at all in the Sangun belt by any petrologists. The mineral assemblages in these schists, especially in the low grade area of the Sangun belt, were considered to be monotonous as compared with those in mafic rocks. However, the authors have recently found lawsonite in quartzofeldspathic schist of the Sangun belt from Shikuma, Shimane prefecture.
As quartzofeldspathic schists distribute most extensively in the Sangun belt although some different lithofacies are recognizable, the consideration on genesis of lawsonite in quartzofeldspathic schists is important to fully understanding of the Sangun metamorphism.
In this paper, the authors will give brief description of occurrence of lawsonite in Shikuma, Gotsu city, Shimane prefecture and a consideration on its genesrs.