[S-MP27] 変形岩・変成岩とテクトニクス

[SMP27-02] Kinetics and Bayesian statistics of zircon growth: an application to detection of pulsed melt fluxes in the Ryoke complex

*宮崎 一博¹、池田 剛²、岩野 英樹^3,4、平田 岳史⁴、檀原 徹³ (1.産業技術総合研究所　地質情報研究部門　、2.九州大学　理学部　地球惑星、3.京都フィッショントラック、4.東京大学　地殻化学実験施設)

キーワード：成長動力学、ミグマタイト、領家、ジルコン

We present the kinetics of zircon growth in migmatite, and detect pulses of melt flux at middle to lower crustal depths beneath a volcanic arc. Migmatites in high-T metamorphic complexes at active continental margins, such as in the Ryoke complex of southwest Japan, are thought to have been produced beneath volcanic arcs. Thermal models suggest that melt advection supplies the heat to form such high-T complexes. We found that zircons in the migmatites of the Ryoke complex were formed by rapid diffusion-controlled growth in multiple stages. The individual growth pulses can be distinguished from each other using a Gaussian mixture model based on Bayesian statistics when the duration of each growth pulse is shorter than the analytical error (1σ) of the zircon age dating, and where the interval between each growth pulse is larger than the analytical error (2σ) of the zircon age dating. This method allows extraction of the growth pulses even when the zircon exhibits incomplete textural evidence for multiple stages of growth. Application of the method to the Ryoke complex revealed three and four pulses of zircon growth in the Mikawa area and five in the Yanai area. The detected major growth pulses are consistent with previously reported major pulses of plutonic activity in the two areas. Therefore, the growth pulses were caused by thermal pulses due to pulses in the melt flux, i.e., melt flux at lower to middle crustal levels. The inferred intervals (2 to 10 Myr apart) of pulsed melt fluxes are similar to those of individual caldera formation in coeval caldera clusters at the surface.