17:15 〜 19:15
[SVC34-P16] Occurrence and characteristics of igneous rocks in Nokobiura, Goto islands
The Goto Islands are located approximately 60 km west of Kyushu. The islands are primarily composed of Miocene sedimentary formations of the Goto Group, as well as acidic volcanic and plutonic rocks (e.g., Kiyokawa et al., 2022). Quaternary basaltic monogenetic volcanoes are scattered across the northern and southern parts of the islands. Additionally, minor igneous dikes and ejecta, the details of which remain unclear, have been observed at various locations along the coastline. In this study, we report the occurrence and characteristics of igneous rocks found in Nokobiura, one of these small igneous bodies, whose details remain poorly understood.
Nokobiura is a bay that opens to the northwest. The innermost part of the bay is characterized by a fault damage zone associated with a northwest–southeast trending fault (F2 fault in Kiyokawa et al., 2022). Distinctly different rock units are exposed on either side of this fault damage zone. Our field survey elucidated the geological characteristics of these rock units, revealing that the northeastern part of the area is mainly composed of sandstone, which is intruded by mafic dikes and sheets with doleritic texture. The whole-rock chemical compositions of the mafic dike and sheet are basalt and basaltic andesite. In contrast, the southwestern part consists of a felsic rock complex composed of rhyolite lava, rhyolitic dikes, and pitchstone dikes. The mafic dikes exhibit a northeast–southwest trend, consistent with the orientation of the F1 normal fault identified by Kiyokawa et al. (2022). Additionally, the pitchstone dikes generally align with the strike of the F2 fault. The potential relationship between the formation of these dikes and fault activity is of particular interest. Fine-grained granite porphyry is observed in proximity to the fault damaged zone. However, the relationship between the granite porphyry and the surrounding rocks remains unclear.
As far as our survey covered, rhyolite lava is widely distributed across the coastal region of the southwestern area. The rhyolite lava is intruded by a northwest-trending rhyolitic dike characterized by flow banding. There are two types of pitchstone dikes, which differ in both distribution and whole-rock chemical composition. One pitchstone dike intrudes closely parallel to the rhyolite dike within the rhyolite lava, while the other intrudes obliquely to both. The former is slightly more felsic than the latter. Both pitchstone dikes are characterized by lithophysal cavities developed along the intrusive contacts.
Our findings suggest that multiple distinct igneous events occurred around Nokobiura,representing different magmatic processes. One event involved mafic intrusions into the Goto Group, while the other possibly reflects the shallow, near-surface internal structure of a rhyolitic volcano.
Nokobiura is a bay that opens to the northwest. The innermost part of the bay is characterized by a fault damage zone associated with a northwest–southeast trending fault (F2 fault in Kiyokawa et al., 2022). Distinctly different rock units are exposed on either side of this fault damage zone. Our field survey elucidated the geological characteristics of these rock units, revealing that the northeastern part of the area is mainly composed of sandstone, which is intruded by mafic dikes and sheets with doleritic texture. The whole-rock chemical compositions of the mafic dike and sheet are basalt and basaltic andesite. In contrast, the southwestern part consists of a felsic rock complex composed of rhyolite lava, rhyolitic dikes, and pitchstone dikes. The mafic dikes exhibit a northeast–southwest trend, consistent with the orientation of the F1 normal fault identified by Kiyokawa et al. (2022). Additionally, the pitchstone dikes generally align with the strike of the F2 fault. The potential relationship between the formation of these dikes and fault activity is of particular interest. Fine-grained granite porphyry is observed in proximity to the fault damaged zone. However, the relationship between the granite porphyry and the surrounding rocks remains unclear.
As far as our survey covered, rhyolite lava is widely distributed across the coastal region of the southwestern area. The rhyolite lava is intruded by a northwest-trending rhyolitic dike characterized by flow banding. There are two types of pitchstone dikes, which differ in both distribution and whole-rock chemical composition. One pitchstone dike intrudes closely parallel to the rhyolite dike within the rhyolite lava, while the other intrudes obliquely to both. The former is slightly more felsic than the latter. Both pitchstone dikes are characterized by lithophysal cavities developed along the intrusive contacts.
Our findings suggest that multiple distinct igneous events occurred around Nokobiura,representing different magmatic processes. One event involved mafic intrusions into the Goto Group, while the other possibly reflects the shallow, near-surface internal structure of a rhyolitic volcano.