5:15 PM - 6:45 PM
[SMP22-P02] Igneous activities of Josoji rhyolite and intrusive rocks of NE Shimane Peninsula, SW Japan: Implication on Miocene volcanism related to back-arc rifting in San’in region
Keywords:basaltic to rhyolitic rocks, petrogenesis, back-arc rifting, Shimane Peninsula, San’in region, SW Japan
The Shimane Peninsula, located in the San'in region of southwestern Japan, preserves an extensive distribution of igneous rocks associated with back-arc rifting during the Miocene. This area is one of the good analogs for understanding igneous activity during the formation of the back-arc basin. Our study focuses on the geochemistry and Sr-Nd isotope systematics of rhyolite lavas (Josoji rhyolite: 18-15 Ma) and the basaltic to dacitic intrusions from Stages I (c 14 Ma) and II (c 13 Ma) in the Shimane Peninsula. These intrusive rocks belong to the medium-K magma series. While the Josoji rhyolite and Stage I rocks exhibit chemical compositions indicative of arc-type signatures, the Stage II rocks display elevated Nb and Ta abundances, suggesting weaker arc signatures.
Geochemical investigation indicates that the Josoji rhyolite originated through partial melting of a mafic crust exhibiting a volcanic arc-type composition. The source material likely shares similarities with the Kori basalts of the Oki Islands, characterized by arc-type low-alkaline tholeiitic basalt formed during the opening of the Japan Sea.
The Stage I rocks (c 14 Ma) exhibit volcanic arc signatures, indicative of their likely formation through the partial melting of an arc-type metasomatized mantle. These interpretations align with previous studies suggesting that some Miocene volcanic rocks in the San'in region have roots in a volcanic arc-type source (Miyake, 1994; Fujibayashi et al., 1989). In contrast, the Stage II rocks (c 13 Ma) are inferred to have originated from the partial melting of a fertile asthenospheric mantle.
During the Miocene in the San'in region, the subduction of the Philippine Sea Plate had not yet occurred (e.g., Kimura et al., 2005). Consequently, the observed volcanic arc-type magmatic activities in the Josoji rhyolite and Stage I rocks likely resulted from the partial melting of a remnant arc-type lithosphere. This lithosphere probably formed before the Miocene, initiated by the subducting Pacific Plate slab beneath southwest Japan before the opening of the Japan Sea (Yamaoka and Wallis, 2023). Conversely, the presence of the more enriched Stage II rocks implies the melting of the upwelling fertile asthenospheric mantle following the volcanic arc-type activities from 18 to 14 Ma.
In conclusion, the magmatic events associated with the Josoji rhyolite and the Stage I and II intrusive rocks describe a diverse array of activities, each occurring independently within the context of thermal events associated with the Japan Sea opening.
Geochemical investigation indicates that the Josoji rhyolite originated through partial melting of a mafic crust exhibiting a volcanic arc-type composition. The source material likely shares similarities with the Kori basalts of the Oki Islands, characterized by arc-type low-alkaline tholeiitic basalt formed during the opening of the Japan Sea.
The Stage I rocks (c 14 Ma) exhibit volcanic arc signatures, indicative of their likely formation through the partial melting of an arc-type metasomatized mantle. These interpretations align with previous studies suggesting that some Miocene volcanic rocks in the San'in region have roots in a volcanic arc-type source (Miyake, 1994; Fujibayashi et al., 1989). In contrast, the Stage II rocks (c 13 Ma) are inferred to have originated from the partial melting of a fertile asthenospheric mantle.
During the Miocene in the San'in region, the subduction of the Philippine Sea Plate had not yet occurred (e.g., Kimura et al., 2005). Consequently, the observed volcanic arc-type magmatic activities in the Josoji rhyolite and Stage I rocks likely resulted from the partial melting of a remnant arc-type lithosphere. This lithosphere probably formed before the Miocene, initiated by the subducting Pacific Plate slab beneath southwest Japan before the opening of the Japan Sea (Yamaoka and Wallis, 2023). Conversely, the presence of the more enriched Stage II rocks implies the melting of the upwelling fertile asthenospheric mantle following the volcanic arc-type activities from 18 to 14 Ma.
In conclusion, the magmatic events associated with the Josoji rhyolite and the Stage I and II intrusive rocks describe a diverse array of activities, each occurring independently within the context of thermal events associated with the Japan Sea opening.