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[SMP28-14] Fluid behavior based on metamorphic reactions in ultramafic rocks from a contact metamorphic zone: An example from the Sanbagawa Belt in the northern Akaishi Mountains
Keywords:ultramafic rocks, Sanbagawa belt, contact metamorphism, Kifune plutonic body, metamorphic reaction, mineral assemblage
Fluid behavior associated with hydration and dehydration reactions of ultramafic rocks in deep subduction zones is closely related to such processes as water circulation and arc volcanism. The distribution of mineral assemblages in ultramafic rocks within high-P metamorphic belts is useful for tracking these fluid processes. However, these high-P metamorphic rocks are strongly affected by retrograde metamorphism, making it difficult to accurately extract information under specific P–T conditions, particularly regarding dehydration reactions that occurred in deep regions. Therefore, in this study, we focus on ultramafic rocks in a contact metamorphic aureole.
The ultramafic rocks of the high-P Sanbagawa metamorphic belt in the northern Akaishi Mountains are interlayered with pelitic schist and are distributed in a narrow, NNE–SSW orientation1. At the northern end, they were affected by the thermal influence of the Kifune plutonic body1. In this study, 25 samples of ultramafic rocks were collected within ~12 km of the intrusion boundary. By analyzing the spatial variations in mineral assemblages and microstructural observations from the non-contact metamorphic zone to the contact metamorphic zone, we examined metamorphic reactions associated with contact metamorphism and investigated the related fluid behavior.
Based on mineral assemblages, the study area was divided into Zone 0 to Ⅲ, with proximity to the intrusion boundary. The primary mineral assemblages were as follows: Zone 0 consists of lizardite, chrysotile, chromian spinel; Zone Ⅰ consists of antigorite, chrysotile, and chromian spinel; Zone Ⅱ consists of olivine and talc; and Zone Ⅲ consists of olivine, tremolite/Mg-hornblende, and spinel. These spatial variations indicate that Zones I to III were affected by contact metamorphism. In addition, the metamorphic reactions can be explained by the MSH system for Zones 0 to II and the CMASH system for Zone III2-4. The reactions defining each zone boundary are as follows: the transition from Zone 0 to Zone I is explained by a phase transformation from lizardite to antigorite; that from Zone I to Zone II is caused by dehydration breakdown of antigorite; and that at the lower boundary of Zone III results from the dehydration breakdown of chlorite, as well as the dehydration of tremolite and Cr–Al spinel. Based on the emplacement pressure conditions of ~1.9 kbar for the Kifune plutonic body5, the temperature conditions can be constrained as follows: Zone I is 350–500 °C, Zone II is 500–630 °C, and Zone III is >720 °C.
Based on the metamorphic reactions, microstructural features, and mineral modal ratios, large-scale and multi-phase fluid activities associated with the intrusion of the Kifune plutonic body are suggested. The transition from the MSH system in Zone II to the CMASH system in Zone III can be explained by the supply of Ca and Al components derived from the Kifune plutonic body. In addition, the presence of olivine-rich veins in Zones II and III indicates the generation of dehydration fluids derived from contact-metamorphosed ultramafic rocks. Furthermore, the occurrence of plagioclase and pargasite in Zone III suggests the supply of Na components from the adjacent pelitic rocks into the ultramafic rocks. These findings indicate that ultramafic rocks not only actively exchanged fluids with surrounding rocks during dehydration stages but also functioned as fluid conduits on a kilometer scale.
Reference
1 Makimoto et al. (1996), GSJ, 34, 114 p.
2 Evans (2004), Int. Geol. Rev., 46, 479-506.
3 Gervilla et al. (2012), Contrib, Mineral, Petrol., 164, 643-657.
4 Kempf et al. (2022), Swiss J. of Geosci., 115, 30 p.
5 Nobuhara et al. (2024), JpGU Meet. Abstr., SCG45-P01.
The ultramafic rocks of the high-P Sanbagawa metamorphic belt in the northern Akaishi Mountains are interlayered with pelitic schist and are distributed in a narrow, NNE–SSW orientation1. At the northern end, they were affected by the thermal influence of the Kifune plutonic body1. In this study, 25 samples of ultramafic rocks were collected within ~12 km of the intrusion boundary. By analyzing the spatial variations in mineral assemblages and microstructural observations from the non-contact metamorphic zone to the contact metamorphic zone, we examined metamorphic reactions associated with contact metamorphism and investigated the related fluid behavior.
Based on mineral assemblages, the study area was divided into Zone 0 to Ⅲ, with proximity to the intrusion boundary. The primary mineral assemblages were as follows: Zone 0 consists of lizardite, chrysotile, chromian spinel; Zone Ⅰ consists of antigorite, chrysotile, and chromian spinel; Zone Ⅱ consists of olivine and talc; and Zone Ⅲ consists of olivine, tremolite/Mg-hornblende, and spinel. These spatial variations indicate that Zones I to III were affected by contact metamorphism. In addition, the metamorphic reactions can be explained by the MSH system for Zones 0 to II and the CMASH system for Zone III2-4. The reactions defining each zone boundary are as follows: the transition from Zone 0 to Zone I is explained by a phase transformation from lizardite to antigorite; that from Zone I to Zone II is caused by dehydration breakdown of antigorite; and that at the lower boundary of Zone III results from the dehydration breakdown of chlorite, as well as the dehydration of tremolite and Cr–Al spinel. Based on the emplacement pressure conditions of ~1.9 kbar for the Kifune plutonic body5, the temperature conditions can be constrained as follows: Zone I is 350–500 °C, Zone II is 500–630 °C, and Zone III is >720 °C.
Based on the metamorphic reactions, microstructural features, and mineral modal ratios, large-scale and multi-phase fluid activities associated with the intrusion of the Kifune plutonic body are suggested. The transition from the MSH system in Zone II to the CMASH system in Zone III can be explained by the supply of Ca and Al components derived from the Kifune plutonic body. In addition, the presence of olivine-rich veins in Zones II and III indicates the generation of dehydration fluids derived from contact-metamorphosed ultramafic rocks. Furthermore, the occurrence of plagioclase and pargasite in Zone III suggests the supply of Na components from the adjacent pelitic rocks into the ultramafic rocks. These findings indicate that ultramafic rocks not only actively exchanged fluids with surrounding rocks during dehydration stages but also functioned as fluid conduits on a kilometer scale.
Reference
1 Makimoto et al. (1996), GSJ, 34, 114 p.
2 Evans (2004), Int. Geol. Rev., 46, 479-506.
3 Gervilla et al. (2012), Contrib, Mineral, Petrol., 164, 643-657.
4 Kempf et al. (2022), Swiss J. of Geosci., 115, 30 p.
5 Nobuhara et al. (2024), JpGU Meet. Abstr., SCG45-P01.