Orthopyroxene from a metabasite of the Ryoke belt in the Ina district, Nagano Prefecture, Central Japan: textural evidence of partial melting.

Abstract

Orthopyroxene-bearing basic metamorphic rock occurs as a block enclosed within pelitic-psammitic metamorphic rock in the Ina district of the Ryoke belt. The orthopyroxene is irregular in shape and includes many rounded crystals of calcic amphibole and plagioclase. Orthopyroxene is a representative metamorphic mineral of the granulite facies for rocks of basic composition and a number of metamorphic reactions to produce orthopyroxene have been proposed. However, none of the metamorphic reactions is consistent with the observed textural relationships and the mineral paragenesis of the rock. The author, therefore, suggests that the orthopyroxene was formed by a distinct process. A simple hypothesis to account for the occurrence of the orthopyroxene is that the orthopyroxene was crystallized after a process of partial melting, i.e., melt produced by partial melting was distributed along grain boundaries of calcic amphibole and plagioclase, and this melt precipitated the orthopyroxene. This hypothesis is supported by the following facts: 1) The texture of the orthopyroxene is unusual for a metamorphic mineral. 2) No simple process other than the crystallization from a partial melt fraction can consistently account for the mode of occurrence of orthopyroxene. 3) Recent experimental results suggest that partial melting of basic rock can take place at the temperatures in the high-grade parts of the Ryoke belt, and furthermore that orthopyroxene is a likely product of such partial melting.