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[SCG52-P26] Origin of plutonic gravels dredged from the South Senoumi-bank, Suruga bay, Japan
Keywords:South Senoumi-bank, Plutonic gravels, Kofu Granitic Complex
Suruga Bay was formed by the subduction of the Philippine Sea Plate on the east into the Eurasian Plate on the west. Suruga Bay can be considered a tectonic valley. In the western part of the mouth of the bay, there are two shoals called "Senoumi", with depths of 46 m and 69 m, respectively, at the shallowest part (Shiba, 2016). These two geomorphic uplifts are referred to as "Kita-tai" for the northern shoal and "Minami-tai" for the southern shoal. The Senoumi Group is distributed in this area, and most of it is thought to have been deposited during the Pleistocene (Okamura et al., 1999). A dredge survey was conducted at South Senoumi-bank in April 2022. In this presentation, we report on the characteristics of the gravels from D01 (water depth: 2,400 m~1,600 m) and D02 (water depth: 2,200 m~1,400 m), including rock description and whole-rock chemical analysis of the gravels, which originate from plutonic rocks.
Two hundred samples obtained from D01 and D02 were classified into sedimentary, volcanic, and plutonic rocks. In D01, 61 % was sedimentary rock, 37 % was volcanic rock, and 2 % was plutonic rock. In D02, sedimentary rock accounted for 76.5 %, volcanic rock for 21 %, and plutonic rock for 2.5 %.
The following plutonic rocks were identified in D01: biotite granite, biotite hornblende granodiorite, biotite hornblende tonalite, biotite monzogranite, orthopyroxene hornblende gabbro, aplite, and granite porphyry. In D02, there were hornblende biotite clinopyroxene tonalite, biotite clinopyroxene orthopyroxene hornblende diorite, biotite clinopyroxene quartz diorite, granophyre, and aplite.
7 of 20 rock samples described were analyzed for whole-rock chemical analysis. The SiO2 content of each sample ranged from 48.51 to 76.81 wt.%, which is very wide composition range. (1) basic rock with SiO2 content around 50 wt.%, (2) intermediate rock with SiO2 content around 65 wt.%, and (3) acid rocks with >75 wt.% were classified into three major groups. These samples were compared with the granitic rocks distributed around Suruga Bay (Ryoke Metamorphic Belt granitoids, Kaikomagatake granitoid pluton, Kofu Granitic Complex, and Tanzawa Tonalite Complex).
(1) 4 of 7 rock samples corresponding to the group of acid rocks with SiO2 content above 75 wt.%. 1 of 4 rock sample belongs to the boundary between the medium K2O series and the high K2O series. And the 3 rock samples belong to the high K2O series. The rocks with characteristics of the high K2O series are the Mizugaki granitic body and the Shosenkyo granitic body in the Kofu Granitic Complex. The Zr contents of the four samples are consistent with those of the Mizugaki granitic body. Based on these results, it is inferred that the four samples are gravels originating from the Mizugaki granitic body.
(2) 2 of 7 rock samples with SiO2 content around 65 wt.%. One of the two samples was not considered due to errors in the data. That is characterized by the Hirose granodioritic body of the Kofu Granitic Complex. The Sanpo Granodioritic body and the Shiodaira Tonalitic body of the Kofu Granitic Complex also show relatively similar compositional ranges, but both belong to the medium K2O series. Therefore, the Hirose Granodioritic body is the source of the samples.
(3) The sample classified as basic rocks with SiO2 contents of around 50 wt.% belong to the boundary between the low and medium K2O series. Takahashi et al. (2004) reported a tonalite with SiO2 content less than 50 wt.% in the Tanzawa Tonalite Complex, and the Fe2O3, Na2O, and CaO contents are like rock in the Tanzawa Tonalite Complex. Therefore, the samples distributed in the low K2O series are presumed to be gravels originating from the Tanzawa Tonalite Complex.
[References]
Okamura, Y et al., (1999): Marine Geology Map Series, 52. Shiba, M (2016) Journal of Fossil Research, 49(1), 3-12. Takahashi, M et al., (2004) Proceedings of the Institute of Natural Sciences Nihon University, 39, 259-284.
Two hundred samples obtained from D01 and D02 were classified into sedimentary, volcanic, and plutonic rocks. In D01, 61 % was sedimentary rock, 37 % was volcanic rock, and 2 % was plutonic rock. In D02, sedimentary rock accounted for 76.5 %, volcanic rock for 21 %, and plutonic rock for 2.5 %.
The following plutonic rocks were identified in D01: biotite granite, biotite hornblende granodiorite, biotite hornblende tonalite, biotite monzogranite, orthopyroxene hornblende gabbro, aplite, and granite porphyry. In D02, there were hornblende biotite clinopyroxene tonalite, biotite clinopyroxene orthopyroxene hornblende diorite, biotite clinopyroxene quartz diorite, granophyre, and aplite.
7 of 20 rock samples described were analyzed for whole-rock chemical analysis. The SiO2 content of each sample ranged from 48.51 to 76.81 wt.%, which is very wide composition range. (1) basic rock with SiO2 content around 50 wt.%, (2) intermediate rock with SiO2 content around 65 wt.%, and (3) acid rocks with >75 wt.% were classified into three major groups. These samples were compared with the granitic rocks distributed around Suruga Bay (Ryoke Metamorphic Belt granitoids, Kaikomagatake granitoid pluton, Kofu Granitic Complex, and Tanzawa Tonalite Complex).
(1) 4 of 7 rock samples corresponding to the group of acid rocks with SiO2 content above 75 wt.%. 1 of 4 rock sample belongs to the boundary between the medium K2O series and the high K2O series. And the 3 rock samples belong to the high K2O series. The rocks with characteristics of the high K2O series are the Mizugaki granitic body and the Shosenkyo granitic body in the Kofu Granitic Complex. The Zr contents of the four samples are consistent with those of the Mizugaki granitic body. Based on these results, it is inferred that the four samples are gravels originating from the Mizugaki granitic body.
(2) 2 of 7 rock samples with SiO2 content around 65 wt.%. One of the two samples was not considered due to errors in the data. That is characterized by the Hirose granodioritic body of the Kofu Granitic Complex. The Sanpo Granodioritic body and the Shiodaira Tonalitic body of the Kofu Granitic Complex also show relatively similar compositional ranges, but both belong to the medium K2O series. Therefore, the Hirose Granodioritic body is the source of the samples.
(3) The sample classified as basic rocks with SiO2 contents of around 50 wt.% belong to the boundary between the low and medium K2O series. Takahashi et al. (2004) reported a tonalite with SiO2 content less than 50 wt.% in the Tanzawa Tonalite Complex, and the Fe2O3, Na2O, and CaO contents are like rock in the Tanzawa Tonalite Complex. Therefore, the samples distributed in the low K2O series are presumed to be gravels originating from the Tanzawa Tonalite Complex.
[References]
Okamura, Y et al., (1999): Marine Geology Map Series, 52. Shiba, M (2016) Journal of Fossil Research, 49(1), 3-12. Takahashi, M et al., (2004) Proceedings of the Institute of Natural Sciences Nihon University, 39, 259-284.