5:15 PM - 7:15 PM
[SVC32-P16] Re-examination of stratigraphy and magmatic processes during the Stage 2 and Stage 3 of Tateyama volcanic activity, central Japan.
Keywords:Tateyama volcano, Tateyama-D tephra, Shomyodaki pyroclastic flow deposit
Tateyama volcano, located in the central part of the Hida Mountains on the border between Toyama and Nagano prefectures, erupted explosively in the Stage 2 of activity at 100 ka, producing pyroclastic flow deposits on the west side of the present Tateyama caldera, which is considered to be the source of the eruption, and pumice on the east side (Harayama et al., 2000, Machida and Arai, 2003). It is known that the proximal facies are subdivided into two; that is, the upper part (Shomyodaki pyroclastic flow: Spfl) consisting of more mafic ejecta (Nogami et al. 2012), associated with magma mixing deposit (Nigorikawa, 2003) and the lower part (Ashikuraji pyroclastic flow: Apfl). However, details such as the onset of magma mixing and whether similar magma mixing occurred at the distant phase remain unclear. The following problems also exist with the Stage 2 ejecta. The Apfl, located the lower unit of the near-phase pyroclastic flow, has been correlated with the Tateyama D tephra (Tt-D), which was ejected during the Stage 2 activity, but the Spfl, located the upper part, has been correlated with Tt-D by Machida and Arai (1979), but it is not known that it was ejected during the Stage 2 activity, because Nogami and Ishizaki (2012) correlated it to the Tateyama E tephra (Tt-E), which was ejected during the Stage 3 of the Tateyama volcanic eruption between 65-70 ka.
In this study, we re-examination the stratigraphic position of the Tt-E during the Stage 2-3, excluding lava flows, based on the comparison with the Spfl, the upper part of the pyroclastic flow deposit in the vicinity of the Tt-E tephra. The chemical compositions of the minerals in the intrinsics are also used to determine the period of magmatic mixing and whether similar magmatic processes can be seen in the ejecta of distant phases. The Tt-D and Tt-E samples were collected near the former Omachi ski resort in the northern part of Omachi City, Nagano Prefecture, and the Apfl and Spfl samples were collected at Raichodaira and the right bank of Joganji River, Ashikuraji, Tateyama Town, Toyama Prefecture. An four outcrops in Ashikuraji, four samples of Apfl were collected from four stratigraphic horizons including the uppermost part. Two samples were taken from the second unit of Spfl. Mineral composition (number ratio) and refractive indices of Opx and Hb were used for correlate, and changes in chemical composition of the core and rim of plagioclase(Pl) and Opx were used to evaluate magma mixing.
The mineral composition of Spfl is composed of light minerals (70%), which is similar to that of the lower Apfl and Tt-D in the distal phases. The heavy minerals in Spfl are Opx, Cpx, and Hb, and biotite is almost absent. These mineralogical characteristics are similar to those of Tt-E. As a result, Spfl has mineral variation of both Tt-D and Tt-E, and it is difficult to determine. The refractive indices of the Opx are generally within the range of 1.710~1.720 except for Tt-E, but only Tt-E shows a higher upper limit of 1.723. In the case of ordinary Hb, while other ejecta fall within the range of 1.681~1.692, Tt-E shows a lower limit of 1.678. Based on these results, it is concluded that Tt-D, Apfl, and Spfl are different from Tt-E, and that both minerals show similar values, and that Spfl should be contrasted with Tt-D, which is a Stage 2 ejecta.
The core-rim chemical compositions of Opx and Hb in the intrinsics show that the compositions of the inverse cumulative zone are found in the distal facies Tt-D, the proximal facies Apfl, and Spfl in the Stage 2 ejecta, and in one of the stratigraphic levels Tt-E in the Stage 3 ejecta. This suggests that the magma mixing identified in Nigorigawa (2003) had already occurred at the beginning of the Stage 2 activity and continued during the Stage 3 activity. In terms of minerals showing reverse zoning, the magnitude of compositional change from the core to the rim indicates that Pl and Opx show particularly large An# and Mg# reverse zoning in the units near the lower and upper edges of Tt-D. In the proximal phases, the An# and Mg# reverse zoning are found in the lower and upper edges of Tt-D, respectively. In the proximal phases, minerals showing especially large An# and Mg# reverse zoning changes were identified in the lower and upper unit of Apfl, and in the lowest unit of Spfl for Pl. These results suggest that magma injection was occurred twice, once at the beginning of the Stage 2 (Apfl eruption) and once at the time of Spfl eruption, which caused a significant change in magma temperature.
In this study, we re-examination the stratigraphic position of the Tt-E during the Stage 2-3, excluding lava flows, based on the comparison with the Spfl, the upper part of the pyroclastic flow deposit in the vicinity of the Tt-E tephra. The chemical compositions of the minerals in the intrinsics are also used to determine the period of magmatic mixing and whether similar magmatic processes can be seen in the ejecta of distant phases. The Tt-D and Tt-E samples were collected near the former Omachi ski resort in the northern part of Omachi City, Nagano Prefecture, and the Apfl and Spfl samples were collected at Raichodaira and the right bank of Joganji River, Ashikuraji, Tateyama Town, Toyama Prefecture. An four outcrops in Ashikuraji, four samples of Apfl were collected from four stratigraphic horizons including the uppermost part. Two samples were taken from the second unit of Spfl. Mineral composition (number ratio) and refractive indices of Opx and Hb were used for correlate, and changes in chemical composition of the core and rim of plagioclase(Pl) and Opx were used to evaluate magma mixing.
The mineral composition of Spfl is composed of light minerals (70%), which is similar to that of the lower Apfl and Tt-D in the distal phases. The heavy minerals in Spfl are Opx, Cpx, and Hb, and biotite is almost absent. These mineralogical characteristics are similar to those of Tt-E. As a result, Spfl has mineral variation of both Tt-D and Tt-E, and it is difficult to determine. The refractive indices of the Opx are generally within the range of 1.710~1.720 except for Tt-E, but only Tt-E shows a higher upper limit of 1.723. In the case of ordinary Hb, while other ejecta fall within the range of 1.681~1.692, Tt-E shows a lower limit of 1.678. Based on these results, it is concluded that Tt-D, Apfl, and Spfl are different from Tt-E, and that both minerals show similar values, and that Spfl should be contrasted with Tt-D, which is a Stage 2 ejecta.
The core-rim chemical compositions of Opx and Hb in the intrinsics show that the compositions of the inverse cumulative zone are found in the distal facies Tt-D, the proximal facies Apfl, and Spfl in the Stage 2 ejecta, and in one of the stratigraphic levels Tt-E in the Stage 3 ejecta. This suggests that the magma mixing identified in Nigorigawa (2003) had already occurred at the beginning of the Stage 2 activity and continued during the Stage 3 activity. In terms of minerals showing reverse zoning, the magnitude of compositional change from the core to the rim indicates that Pl and Opx show particularly large An# and Mg# reverse zoning in the units near the lower and upper edges of Tt-D. In the proximal phases, the An# and Mg# reverse zoning are found in the lower and upper edges of Tt-D, respectively. In the proximal phases, minerals showing especially large An# and Mg# reverse zoning changes were identified in the lower and upper unit of Apfl, and in the lowest unit of Spfl for Pl. These results suggest that magma injection was occurred twice, once at the beginning of the Stage 2 (Apfl eruption) and once at the time of Spfl eruption, which caused a significant change in magma temperature.