5:15 PM - 6:45 PM
[SMP21-P03] Genesis of the enriched volcanic rocks at Sapporo-dake and Soranuma-dake in southwestern Hokkaido: Insights into the formation of the Toyoha polymetallic deposit
Keywords:Sr-Nd-Pb isotopes, NE Japan arc , hydrothermal deposits, trace elements
The variety of igneous activity reflects the diversity of processes occurring within the Earth's interior and has a significant impact on the geology, geography, and resource availability of a region. For instance, in southwestern Hokkaido, Japan, Pliocene calc-alkaline igneous activity initiated the formation of some of the major ore deposits, such as the Toyoha polymetallic deposit [1]. However, information on Pliocene igneous activity in southwestern Hokkaido is limited, and the relationship between igneous activity and the formation of ore deposits remains unclear. In this study, we analyzed the chemical compositions and Sr-Nd-Pb isotopic ratios of the Pliocene volcanic rocks (Muine, Nagao, Yoichi, Sapporo, and Soranuma) in southwestern Hokkaido to investigate the origin of these igneous rocks.
The volcanic rocks of Muine, Nagao, and Yoichi exhibit depleted Sr-Nd-Pb isotope ratios similar to those of the Quaternary basalts of southwestern Hokkaido [2]. In contrast, the volcanic rocks of Sapporo and Soranuma displayed more enriched Sr-Nd-Pb isotopic ratios, with Sapporo basalts showing enriched Nd isotopic ratios comparable to the southwest Hokkaido basalts prior to 15 Ma (143Nd/144Nd < 0.5128 [3]). The Zr/Nb ratios (with respect to SiO2 concentrations) were higher in Muine, Nagao and Yoichi, and lower in Sapporo and Soranuma. These trends in isotopic ratios and concentrations are consistent with those reported in a previous study [4].
The pre-15 Ma basalts in southwestern Hokkaido were formed by partial melting of the sub-continental lithospheric mantle (SCLM) with enriched Nd isotopic ratios [3]. After 15 Ma, magmas in southwestern Hokkaido originated from the asthenospheric mantle with depleted Nd isotopic ratio (143Nd/144Nd > 0.5128 [3]). Therefore, the enriched Nd isotopic ratios observed at the Sapporo and Soranuma are unique and do not match the isotopic ratios typically seen in igneous activities after the Japan Sea opening. Possible enriched source materials that mixed into Sapporo basalt include materials derived from subducting slabs (i.e., fluids or melts derived from sediments or oceanic crust) and SCLM-derived materials remaining beneath the Hokkaido Island. However, explaining the difference in Zr/Nb ratios between the basalts of Yoichi and Sapporo solely by changing the contribution from slab-derived materials is difficult. Therefore, it is likely that the SCLM-derived materials with low Zr/Nb ratios, similar to those that formed the volcanic rocks before 15 Ma, are present in the Sapporo basalts.
In the tectonic setting of the Pliocene southwest Hokkaido, the SCLM is not under P-T conditions that would partially melt. To form the Sapporo basalts, SCLM-derived materials have to be transported to the partial melting region in the mantle wedge. Previous studies indicate that seamounts formed before the Erimo seamount are currently being subducted beneath southwestern Hokkaido [5]. This may promote structural erosion of the upper plate and the formation of a mélange containing SCLM material. The SCLM fragments may have been serpentinized by the fluids dehydrated from the slab, causing them to gain buoyancy and rise as rock diapirs, which then partially melted [6]. In the presentation, we will also explore the potential importance of diverse source materials in the formation of the in the primary magma which formed the Toyoha polymetallic deposit.
[1] Watanabe 2002. Resour. Geol. 52, 191-210.
[2] Nakamura et al., 2019. Gondwana Res. 70, 36-49.
[3] Takanashi et al., 2011. Lithos 125, 368-392.
[4] Nakagawa et al., 2013. 130th Annual Meeting of the Geol. Soc. Japan, R3-O-3.
[5] Yamazaki, Okamura, 1989. Tectonophysics 160, 207-229.
[6] Marschall, Schumacher, 2012. Nat. Geosci. 5, 862-867.
The volcanic rocks of Muine, Nagao, and Yoichi exhibit depleted Sr-Nd-Pb isotope ratios similar to those of the Quaternary basalts of southwestern Hokkaido [2]. In contrast, the volcanic rocks of Sapporo and Soranuma displayed more enriched Sr-Nd-Pb isotopic ratios, with Sapporo basalts showing enriched Nd isotopic ratios comparable to the southwest Hokkaido basalts prior to 15 Ma (143Nd/144Nd < 0.5128 [3]). The Zr/Nb ratios (with respect to SiO2 concentrations) were higher in Muine, Nagao and Yoichi, and lower in Sapporo and Soranuma. These trends in isotopic ratios and concentrations are consistent with those reported in a previous study [4].
The pre-15 Ma basalts in southwestern Hokkaido were formed by partial melting of the sub-continental lithospheric mantle (SCLM) with enriched Nd isotopic ratios [3]. After 15 Ma, magmas in southwestern Hokkaido originated from the asthenospheric mantle with depleted Nd isotopic ratio (143Nd/144Nd > 0.5128 [3]). Therefore, the enriched Nd isotopic ratios observed at the Sapporo and Soranuma are unique and do not match the isotopic ratios typically seen in igneous activities after the Japan Sea opening. Possible enriched source materials that mixed into Sapporo basalt include materials derived from subducting slabs (i.e., fluids or melts derived from sediments or oceanic crust) and SCLM-derived materials remaining beneath the Hokkaido Island. However, explaining the difference in Zr/Nb ratios between the basalts of Yoichi and Sapporo solely by changing the contribution from slab-derived materials is difficult. Therefore, it is likely that the SCLM-derived materials with low Zr/Nb ratios, similar to those that formed the volcanic rocks before 15 Ma, are present in the Sapporo basalts.
In the tectonic setting of the Pliocene southwest Hokkaido, the SCLM is not under P-T conditions that would partially melt. To form the Sapporo basalts, SCLM-derived materials have to be transported to the partial melting region in the mantle wedge. Previous studies indicate that seamounts formed before the Erimo seamount are currently being subducted beneath southwestern Hokkaido [5]. This may promote structural erosion of the upper plate and the formation of a mélange containing SCLM material. The SCLM fragments may have been serpentinized by the fluids dehydrated from the slab, causing them to gain buoyancy and rise as rock diapirs, which then partially melted [6]. In the presentation, we will also explore the potential importance of diverse source materials in the formation of the in the primary magma which formed the Toyoha polymetallic deposit.
[1] Watanabe 2002. Resour. Geol. 52, 191-210.
[2] Nakamura et al., 2019. Gondwana Res. 70, 36-49.
[3] Takanashi et al., 2011. Lithos 125, 368-392.
[4] Nakagawa et al., 2013. 130th Annual Meeting of the Geol. Soc. Japan, R3-O-3.
[5] Yamazaki, Okamura, 1989. Tectonophysics 160, 207-229.
[6] Marschall, Schumacher, 2012. Nat. Geosci. 5, 862-867.