Magmatic plumbing system of the 3400 BC caldera-forming eruption (Numazawako eruption) of Numazawa volcano as deduced by componentry and whole-rock and mineral compositions of the pyroclastic deposits
BC
AbstractThe BC caldera-forming eruption of Numazawa volcano (the Numazawako eruption), NE Japan, began with a cataclysmic pyroclastic flow (phase I) followed by a Plinian eruption, phreatomagmatic eruptions, and a second Plinian eruption (phases II-IV, respectively). Petrological examinations revealed that a variety of juvenile pyroclasts were ejected during the eruption. White pumice (WP) rich in euhedral phenocrysts ( . -. wt.% SiO ) dominated the juvenile material expelled during the two early eruption phases, suggesting that WP-forming dacitic magma (~ -°C) constituted the main and upper portions of the pre-eruptive magma chamber. Gray pumice, rich in crystal fragments ( . -. wt.% SiO ), was a minor component of this eruption, produced by the mechanical breakdown of phenocrysts as the WP magma ascended in the conduit. Wholerock compositions of scoria suggest that two different mafic magmas, a low-Ba type (LBa; < wt.% SiO , > °C) and a high-Ba (HBa) type (< . wt.% SiO , > °C), were injected separately into the magma chamber. Diffusion profiles of dacite-derived magnetite phenocrysts in black scoria (BS; -. wt.% SiO ) show that the Numazawako eruption was triggered by the injection of LBa magma. LBa magma mixed with dacite magma in the chamber and erupted as BS during the two early eruption phases. In contrast, gray scoria (GS; . -. wt.% SiO ) has a distinct chemical composition found only during the two later eruption phases. The abundance and whole-rock composition of GS suggest that injection of HBa magma occurred immediately before or during phase III; the HBa magma mixed with BS-forming magma in the chamber to produce the GS magma. This second magma injection probably resulted in over-pressurization of the chamber, thereby triggering the second Plinian eruption.
K Ar age of a rhyolite lava (Kohideyama rhyolite) on the northwestern flank of Mt. Kohide, Nakatsugawa City, Gifu Prefecture (Yasuo ISHIZAKI) * (Ken ichi YASUE) ** Mt. Kohide (Nakatsugawa City, Gifu Prefecture) is located 7 km north of the Atera fault. A partial collapse of the northwestern flank of Mt. Kohide has exposed a 5 m wide dyke like conduit 15 30 m beneath the extrusive rhyolite lava that was fed by it. The rocks of the extrusive lava and the feeder are nearly aphyric (< 0.5 vol% phenocrysts), biotite rhyolite (73 wt% whole rock SiO 2). The extrusive lava yields a K Ar age of 23.01 0.36 Ma. Within the measurement errors, this age coincides with the previously determined K Ar ages of biotite rhyolite dykes intruded along the Atera fault (23 22 Ma) and the rhyolitic welded tuff of the Hachiya formation (40 km southwest of Mt. Kohide; 22 Ma) as well as the K Ar and fission track ages of the rift type alkalic moonstone rhyolites from the Hokuriku area (25 22 Ma). This suggests that rhyolitic volcanism occurred contemporaneously in central Japan between the Late Oligocene and Early Miocene.
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