15:30 〜 17:00
[SMP25-P01] Igneous Activities During Middle Miocene Back-arc Rifting in San’in Region: A Study from Josoji Formation and Neogene Intrusive Rocks of NE Shimane Peninsula, SW Japan
キーワード：Back-arc basin rifting, Intrusive rocks, Geochemistry, Josoji Formation rhyolite, Sediment contrubution, Shimane Peninsula
The middle Miocene volcanism in San'in region represents the rifting stage of back-arc basin (BAB) of Southwest Japan during the opening of Japan Sea. In Northeastern Shimane peninsula, remarkable subalkalic volcanisms were recovered from several formations including the rhyolitic Josoji Formation (Josoji rhyolites) and following Basic-Intermediate intrusive rocks, which were divided into three stratigraphic stages (Stage I-III basalts). Previous works have shown that Miocene San'in region magmatism has a transitional nature between arc-like and mid-ocean-ridge (MOR)-like volcanisms and speculate them to originate from a metasomatized arc-type lithosphere. As there is no evidence proposed, it is important to provide firm evidence for their magmatism as a progression toward understanding the magmatic system in the BAB rifting of San'in region.
Petrographic, geochemical, and Sr-Nd isotope analyses were carried out for Josoji rhyolites and Basic-Intermediate intrusive rocks from Stage I and Stage II basalts. The petrographic observation of Josoji rhyolites samples revealed rhyolite lava and pyroclastic rocks, and the samples from Stage I and Stage II basalts show basaltic to dacitic rocks. All samples show whole-rock chemical composition, ranging between tholeiite and calc-alkaline. They have an arc-like rock signature with enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs) and rear-earth elements (REEs). Josoji rhyolites and Stage I basalts have a distinctive strong depletion in HFSEs similar to normal MORB, whereas Stage II basalts are slightly more enriched in incompatible elements with a weak depletion in HFSEs compared to typical arc-like basalt including some positive Zr-Hf anomalies. All samples show a narrow range of initial 87Sr/86Sr ratio between 0.7042 and 0.7051 and εNd(t) between +2.16 and +3.92.
For Josoji rhyolites and Stage I basalts, the geochemical and Sr-Nd isotopic data suggest that Josoji rhyolites could be produced by crustal materials that are geochemically resembling those from Stage I basalts, which is supported by partial melting modeling. The geochemical heterogeneity between Stage I and Stage II basalts and the similarities in their Sr-Nd isotopic composition can be explained upon sediments' contribution in the magmatism. The following mechanisms are the possible explanation for the two stages basalts heterogeneity. They can form by two different degrees of partial melting of a metasomatized lithosphere by a sediment melt, in which a higher and lower degree of melting produced Stage I and Stage II basalts respectively. Another possibility is by a common source mantle contaminated by continental crust in two different ratios. Further examination will be considered to clarify the forming mechanism.
Petrographic, geochemical, and Sr-Nd isotope analyses were carried out for Josoji rhyolites and Basic-Intermediate intrusive rocks from Stage I and Stage II basalts. The petrographic observation of Josoji rhyolites samples revealed rhyolite lava and pyroclastic rocks, and the samples from Stage I and Stage II basalts show basaltic to dacitic rocks. All samples show whole-rock chemical composition, ranging between tholeiite and calc-alkaline. They have an arc-like rock signature with enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs) and rear-earth elements (REEs). Josoji rhyolites and Stage I basalts have a distinctive strong depletion in HFSEs similar to normal MORB, whereas Stage II basalts are slightly more enriched in incompatible elements with a weak depletion in HFSEs compared to typical arc-like basalt including some positive Zr-Hf anomalies. All samples show a narrow range of initial 87Sr/86Sr ratio between 0.7042 and 0.7051 and εNd(t) between +2.16 and +3.92.
For Josoji rhyolites and Stage I basalts, the geochemical and Sr-Nd isotopic data suggest that Josoji rhyolites could be produced by crustal materials that are geochemically resembling those from Stage I basalts, which is supported by partial melting modeling. The geochemical heterogeneity between Stage I and Stage II basalts and the similarities in their Sr-Nd isotopic composition can be explained upon sediments' contribution in the magmatism. The following mechanisms are the possible explanation for the two stages basalts heterogeneity. They can form by two different degrees of partial melting of a metasomatized lithosphere by a sediment melt, in which a higher and lower degree of melting produced Stage I and Stage II basalts respectively. Another possibility is by a common source mantle contaminated by continental crust in two different ratios. Further examination will be considered to clarify the forming mechanism.