[S-CG48] Petrology, Mineralogy & Resource Geology

[SCG48-P13] Eruptibility of Japanese felsic plutons

*Hiroaki Sato¹, Yoshiaki Tainosho¹, Tatsuo Kanamaru², Toshio Arai¹ (1.Kobe University, 2.Nihon University)

Keywords:felsic plutons, eruptibility, viscosity coefficient, shallow intrusion, degassing and crystallization

Scaillet et al. (1998) and Takeuchi (2011) showed that bulk viscosities of erupted magmas are generally less than 10⁶ Pa s in the magma reservoirs. This paper examines the eruptibility of Japanese felsic plutons in terms of bulk viscosity less than 10⁶ Pa s in the reservoir. Viscosity of water-saturated minima melts in P-T space is presented by Glazner (2014), where viscosities more than 10⁶ Pa s are observed at less than 780 C for 0.1 GPa (corresponding to 3-4 km depth). Our calculations of bulk viscosity and density for typical Yakushima granite (YMG of Anma et al., 1998, SiO₂ = 70.92 wt%) by using rhyolite-MELTS (Gualda et al., 2012), viscosity equation of Giordano et al. (2008) and density equation of Ochs and Lange (1997) yielded more than 10⁶ Pa s of bulk viscosity of water-saturated granitic magma at less than 820 C and 0.1 GPa. Many felsic plutons show evidence of shallow intrusions, where vesiculation and concomitant degassing cause crystallization and increase of bulk viscosity leading to less eruptible conditions. Among plutons in Japanese islands, many are thought to have emplaced in shallow levels (less than 3-4 km) according to a) drill holes penetrating hot felsic plutons, b) plutons intruding into contemporaneous volcanic strata, c) porphyritic texture with fine-grained matrix, d) presence of druses, aplites, and pegmatites. From these pieces of evidence, we suggest that many Japanese felsic plutons are not eruptible after the final emplacement.