Three-dimensional cooling pattern of a granitic pluton, 1; The Study of exsolution sub-solidus reactions in the Toki granite, Central Japan

Yuguchi, Takashi; Tsuruta, Tadahiko  ; Nishiyama, Tadao*

This paper presents results of petrographical studies examining development and variation of sub-solidus reactions recorded in the Toki granite, Central Japan, which show the three-dimensional cooling pattern of this zoned pluton. Samples collected from 19 boreholes in the Toki granite display characteristics indicative of spatial variation in the extent of sub-solidus reactions. Exsolution coarsening has produced microperthite textures with albite - rich lamella in this pluton, whereas deuteric coarsening has resulted in patchperthite with albite - rich patch. The width and spacing of albite - rich lamella in microperthite increase systematically and prominently with elevation in the pluton, indicating that the Toki granite effectively cooled from the roof during the exsolution coarsening stage. The hornblende - plagioclase and ternary feldspar thermometers indicate a temperature range of 780 to 690 C in which volume diffusion took place, associated with exsolution coarsening.