Three-dimensional cooling pattern of a granitic pluton, 2; The Study of deuteric sub-solidus reactions in the Toki granite, Central Japan

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

Petrographical studies examining the development and variations of sub-solidus reactions recorded in the Toki granite represent the three-dimensional cooling pattern of this zoned pluton in Central Japan. Samples collected from 19 boreholes in the Toki granite show characteristics indicative of spatial variations in the extent of sub-solidus reactions. Exsolution coarsening has produced microperthite including albite-rich lamellae in this pluton, while deuteric coarsening has resulted in patchperthite, myrmekite, and the reaction rim. The extent of deuteric coarsening reactions can be evaluated by the width and spacing of albite-rich patch in patchperthite, and by the thickness of myrmekite and the reaction rim. The width, spacing and thickness of these textural features increase systematically with elevation and also increase gradually in the horizontal inward in the western part but not in the eastern part of the pluton. The systematic variations in textural development indicate that the Toki granite cooled effectively from the roof and from the western margin during the deuteric coarsening stage. The deuteric coarsening may have occurred at temperatures below 500C based on ternary feldspar thermometry.