5:15 PM - 6:45 PM
[SCG45-P01] Estimation of emplacement pressure of plutonic rocks around the Itoigawa–Shizuoka Tectonic Line and reconstruction of geological distribution before the fault movement
Keywords:Itoigawa-Shizuoka Tectonic Line, hornblende geobarometry, Kifune plutonic body, Shimosuwa plutonic body, Chino plutonic body, reconstruction of geological distribution
A comparison result in the thermal structure of metamorphic rocks around the Itoigawa-Shizuoka Tectonic Line (ISTL), one of the largest faults on land in Japan, suggests that two felsic plutonic bodies (Shimosuwa and Kifune bodies) located more than 10 km apart from each other along the ISTL constituted a single plutonic body before the ISTL faulting. Two more felsic plutonic bodies (Chino and Kaikomagatake bodies) that were intruded during the Miocene at about the same time as the Shimosuwa and Kifune bodies are also distributed along the ISTL. Clarifying the petrological relationship between these four plutonic bodies helps verify the reconstructed geological distribution, that is, the total displacement of ISTL faulting based on the analysis of metamorphic rocks.
In this study, we estimated the emplacement pressure conditions of the Kifune, Shimosuwa, and Chino bodies using Al-in-hornblende geobarometry with a focus on the similarity that all these felsic plutonic bodies contain hornblende. For the pressure estimations, the temperature conditions were also calculated using a Ti-in-hornblende geothermometry, and only the data plotted near the solidus curve on the P-T diagram were used. We then compared them, incorporating the pressure data of the Kaikomagatake body obtained from a previous study.
The results indicate the emplacement pressure conditions of ~1.9 kbar for the Kifune body and ~1.7 kbar for the Shimosuwa and Chino bodies, showing a similarity between the three bodies. In contrast, the emplacement pressure condition of the Kaikomagatake body is ~2.2–2.4 kbar, which is distinctly different from the other three bodies. These support the idea, showing that the Kifune and Shimosuwa bodies constituted a single plutonic body before the ISTL faulting, based on the analysis of metamorphic rocks. According to the metamorphic rocks study, the Chino and Kaikomagatake bodies were spatially close to each other before faulting. However, considering the pressure conditions, the Chino body was not the same as the Kaikomagatake body but the same as the Kifune and Shimosuwa bodies, suggesting that the "Kifune-Shimosuwa-Chino" plutonic body, which is comparable in size to the Kaikomagatake plutonic body existed before ISTL faulting. This idea is consistent with the fact that the presently recognized contact metamorphic aureole around the Kifune body is extensive compared to the scale of the plutonic body and is elongated along the ISTL.
In this study, we estimated the emplacement pressure conditions of the Kifune, Shimosuwa, and Chino bodies using Al-in-hornblende geobarometry with a focus on the similarity that all these felsic plutonic bodies contain hornblende. For the pressure estimations, the temperature conditions were also calculated using a Ti-in-hornblende geothermometry, and only the data plotted near the solidus curve on the P-T diagram were used. We then compared them, incorporating the pressure data of the Kaikomagatake body obtained from a previous study.
The results indicate the emplacement pressure conditions of ~1.9 kbar for the Kifune body and ~1.7 kbar for the Shimosuwa and Chino bodies, showing a similarity between the three bodies. In contrast, the emplacement pressure condition of the Kaikomagatake body is ~2.2–2.4 kbar, which is distinctly different from the other three bodies. These support the idea, showing that the Kifune and Shimosuwa bodies constituted a single plutonic body before the ISTL faulting, based on the analysis of metamorphic rocks. According to the metamorphic rocks study, the Chino and Kaikomagatake bodies were spatially close to each other before faulting. However, considering the pressure conditions, the Chino body was not the same as the Kaikomagatake body but the same as the Kifune and Shimosuwa bodies, suggesting that the "Kifune-Shimosuwa-Chino" plutonic body, which is comparable in size to the Kaikomagatake plutonic body existed before ISTL faulting. This idea is consistent with the fact that the presently recognized contact metamorphic aureole around the Kifune body is extensive compared to the scale of the plutonic body and is elongated along the ISTL.