5:15 PM - 6:30 PM
[SVC48-P09] Petorologic Study of the Stage IV Eruptives of Myoko Volcano
Keywords:Myoko Volcano, magma mixing, magma system
The stage IV, the youngest stage of Myoko Volcano, consists of three sub-stages, i.e., pre-collapse (43 ka-), collapse (21 ka-6 ka), and post-collapse stages (6 ka-present) (stage names are modified from those in H08). The volcanic activity during the pre-collapse sub-stage formed the dacitic to andesitic Shibutamigawa Pyroclastic Flow Deposit (SPFD; 55.2-62.9 wt.% SiO2; 42190 ± 380 yr BP) and the directly overlying, basaltic Nishikawadani Scoria Flow Deposit (NSFD; 50.5-52.8 wt.% SiO2). (14C age is from 08). The SPFD shows a hydrous phenocryst assemblage (Pl + Amp + Opx + Cpx ± Ol ± Opq) whereas the NSFD shows an anhydrous phenocryst assemblage (Pl ± Opx + Cpx + Ol ± Opq). The volcanic activity during the central cone stage formed two andesitic to dacitic pyroclastic flow deposits, i.e., the Akakura Pyroclastic Flow Deposit(APFD; 5510 ± 70 yr. BP, 60.3-64.4 wt.% SiO2)and the Ohtagirigawa Pyroclastic Flow Deposit (OPFD; 4060 ± 60 yr. BP, 56.9-64.1 wt.% SiO2) (14C ages are from H08). The APFD and OPFD show hydrous phenocryst assemblage (Pl + Amp + Opx ± Cpx ± Ol ± Qtz ± Opq) and contain quenched mafic enclaves (55.2-57.1 wt.% SiO2; phenocryst assemblage Pl ± Amp + Opx + Cpx ± Ol ± Opq). Eruptives of pre- and post-collapse sub-stages show distinct linear mixing trends on many Harker diagrams. Although major and trace elements are broadly similar in all dacites erupted between pre- and post-collapse stages, there are slight differences in K2O contents. In addition, the mafic magmas erupted between pre- and post-collapse stages are differ in TiO2, Al2O3, K2O, P2O5, V,and Sr contents. These petrological features indicate that the eruptions of the pre- and post-collapse sub-stages were fed by different and transitory silicic magma chambers, repeatedly recharged by mafic magmas of diverse compositions.