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

Abstract

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 the sub -solidus reactions. Exsolution coarsening has produced microperthite, including albite -rich lamellae, in this pluton, while deuteric coarsening has resulted in the formation of patchperthite, myrmekite, and the reaction rim. The extent of the deuteric coarsening reactions can be evaluated from the width and spacing of the albiterich patch in patchperthite and from the thickness of myrmekite and the reaction rim. The width, spacing, and thickness of these textural features increase systematically with elevation; they also increase gradually in the horizontal inward direction in the western part of the pluton 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 500 °C, as indicated by ternary feldspar thermometry.

The spatial variation of initial 87Sr/86Sr ratios in the Toki granite, Central Japan: Implications for the intrusion and cooling processes of a granitic pluton

“…During the exsolution stage (780 -690 °C), the Toki granite was effec- Yuguchi et al, 2011a), whereas during the deuteric stage (below 500 °C) cooling was from both the roof and western margin (Figs. 3, 4, 5 and 6 of Yuguchi et al, 2011b). Multi -sampling thermochronology based on plural mineral dating with different closure temperatures therefore is a good indicator of the cooling behavior in a granitic pluton (Yuguchi et al, 2011c).…”

“…3 and 4 of Yuguchi et al, 2011a), patch perthite, myrmekite and reaction rims (Figs. 3, 4, 5 and 6 of Yuguchi et al, 2011b) in plutonic rocks are a reliable tool for quantifying the cooling process because the development of sub -solidus reaction textures reflects the duration of annealing during which sub -solidus coarsening reactions occurred (Yuguchi and Nishiyama, 2007). Two sub -solidus reaction stages-exsolution and deuteric reaction stages-have been identified in the Toki granitic pluton.…”

“…5A and B of Yuguchi et al, 2011c). Yuguchi et al (2011bYuguchi et al ( , 2011c indicated that the Toki granite effectively cooled from the roof and the western margin during the sub -solidus deuteric stage (below 500 °C), biotite K -Ar closure stage (300 ± 50 °C) and zircon fission -track closure stage (240 ± 50 °C). That is, cooling started from the western upper margin of the Toki granite (Table 2).…”

“…The combination of petrography based on sub -solidus reaction texture (Yuguchi et al, 2011a(Yuguchi et al, , 2011b) and multisample thermochronology, including biotite K -Ar and zircon fission -track dating (Yuguchi et al, 2011c), revealed the cooling process of the Toki granitic pluton from solidus to zircon fission -track closure temperature.…”

“…The cooling process of the Toki granite in the subsolidus temperature range between the solidus (about 780 °C) and zircon fission -track closure (240 ± 50 °C) is described based on petrography (the extent of sub -solidus reaction textures; Yuguchi et al, 2011aYuguchi et al, , 2011b) and multisample thermochronology (biotite K -Ar ages and zircon fission -track ages; Yuguchi et al, 2011c).…”