Mineral parageneses of argillaceous xenoliths in andesitic rocks from the Nijo-san and Amataki-yama districts, Sourthwest Japan.

Abstract

The argillaceous xenoliths within the andesitic lavas of the N??jo-san and Amatakiyama districts were recrystallized successively by the thermal effects of the magma. The mineral assemblages of these xenoliths can be classified into the large-grained and fine-grained groups. The former is subdivided into the staurolite-bearing and staurolitefree assemblages. The staurolite-bearing parageneses of the minerals can be regarded as the amphibolite or pyroxene hornfels facies and the stauro lite-free ones composed of minerals with large grain-size belong to the pyroxene hornfels facies. Such fine-grained parageneses as the corona texture and the breakdown products of garnet were formed under the conditions of the pyroxene hornfels or sanidinite facies. From the progressive change of mineral facies and the distinct descrepancy of grain-size between the large mineral assemblages and fine mineral ones, it is possible to consider, roughly estimating, that the former indicates the stage of contact metamorphism and the latter the stage of pyrometamorphism.