5:15 PM - 7:15 PM
[SCG53-P06] Magma processes of volcano-plutonic complex from the Tokuyama Bay area in the south-central part of Yamaguchi Prefecture
Keywords:Late Mesozoic, Volcano-plutonic complex, Mt. Taikazan, Tokuyama Bay
Cretaceous felsic (or silicic) volcanic rocks are widely distributed in the inner zone of Southwest Japan. This igneous activity has been regarded as a major process in the formation of the crust on active continental margin (Imaoka et al., 2019). A large-scale felsic pyroclastic flow deposit is exposed in the West Chugoku area at about 95-85 Ma (Imaoka et al., 2019). However, few examples of volcano-plutonic complex occur in the Seto Inland Sea area. The Taikazan area on the Oshima Peninsula facing Tokuyama Bay is underlain by Cretaceous effusive rocks and intrusive rocks. The combination of such lithofacies can be regarded as a volcano-plutonic complex. The results of this study provide reveal the formation process of Cretaceous large igneous activities called ignimbrite flare-up in active continental margins. In this study, we describe the characteristics of effusive rocks and intrusive rocks and discuss their formation history.
The Tokuyama Bay area in the south-central Yamaguchi Prefecture is composed of metamorphic rocks and is unconformably overlaid on the Cretaceous volcanic edifices, which are andesitic rocks in the lower sequences and dacitic rocks in the upper sequences, intercalated with thin pelitic layers around the boundary between andesitic and dacitic edifices. The edifices consist of lavas and pyroclastic rocks, such as tuff and volcanic tuff based on constituent materials. The distribution of effusive rocks is solely exposed in limited to the Taikazan area. The granites intrude into the effusive rocks and metamorphic rocks in the southwestern part of the Taikazan area, Sukumojima, Otsushima, Kurokamijima, and Senjima in the Tokuyama Bay area. Unsorted breccia occurs significantly between the basement rocks and the Cretaceous edifices. The outcrops of unsorted breccia layer are located in limited areas, close to the unconformable boundaries. These unsorted breccias are composed of broken crystalline schist with a small amount of pyroclastic breccia.
The formation history was examined based on the above lithologic classification and geological structure. At first, a volcano-clastic basin structure is formed by subsidence of the crystalline schist due to a normal fault. The unsorted breccias deposit along the fault. Then, andesitic magma erupts and deposits in the fault related basin to form a caldera. The volcanic edifices, with andesitic and dacitic compositions, locally overlie the crystalline schist. The granite intrudes into all of them and is disclosed by fault movement in the western part of the study area.
The Tokuyama Bay area in the south-central Yamaguchi Prefecture is composed of metamorphic rocks and is unconformably overlaid on the Cretaceous volcanic edifices, which are andesitic rocks in the lower sequences and dacitic rocks in the upper sequences, intercalated with thin pelitic layers around the boundary between andesitic and dacitic edifices. The edifices consist of lavas and pyroclastic rocks, such as tuff and volcanic tuff based on constituent materials. The distribution of effusive rocks is solely exposed in limited to the Taikazan area. The granites intrude into the effusive rocks and metamorphic rocks in the southwestern part of the Taikazan area, Sukumojima, Otsushima, Kurokamijima, and Senjima in the Tokuyama Bay area. Unsorted breccia occurs significantly between the basement rocks and the Cretaceous edifices. The outcrops of unsorted breccia layer are located in limited areas, close to the unconformable boundaries. These unsorted breccias are composed of broken crystalline schist with a small amount of pyroclastic breccia.
The formation history was examined based on the above lithologic classification and geological structure. At first, a volcano-clastic basin structure is formed by subsidence of the crystalline schist due to a normal fault. The unsorted breccias deposit along the fault. Then, andesitic magma erupts and deposits in the fault related basin to form a caldera. The volcanic edifices, with andesitic and dacitic compositions, locally overlie the crystalline schist. The granite intrudes into all of them and is disclosed by fault movement in the western part of the study area.