Three-dimensional cooling pattern of a granitic pluton I: The study of exsolution sub-solidus reactions in the Toki granite, Central Japan

Abstract

This paper presents the results of petrographical studies carried out to examine the development and variation of sub-solidus reactions occurring in the Toki granite, Central Japan. The results reveal the three-dimensional cooling pattern of this zoned pluton. Samples collected from 19 boreholes in the Toki granite indicate a spatial variation in the extent of sub-solidus reactions. Exsolution coarsening produced microperthite textures with albite-rich lamellae in this pluton, whereas deuteric coarsening resulted in patchperthite with albite-rich patches. The width and spacing of the albite-rich lamella in microperthite increase systematically and prominently with elevation in the pluton. This indicates that the Toki granite effectively cooled from the roof during the exsolution coarsening stage. Measurements obtained using the hornblende-plagioclase and ternary feldspar thermometers indicate that volume diffusion took place in the temperature range 780-690 °C; this diffusion was associated with exsolution coarsening.