Presentation Information
[R7P-03]Preliminary investigation of geochemical characteristics of basement rocks in Southwest Japan for petrogenetic studies of arc magmas
*Takehiro HIRAYAMA1, Masako Yoshikawa1, Tomoyuki Shibata1, Kenta Kawaguchi1, Masaaki Owada2 (1. Hiroshima University, 2. Yamaguchi University)
Keywords:
Southwest Japan Arc,Basement rock,Geochemistry
In general, the chemical compositions of arc magmas are considered to be modified by multiple processes, including the addition of slab-derived components, fractional crystallization, magma mixing, and assimilation of crustal materials, in addition to the primary characteristics of magmas generated by partial melting of the mantle wedge (e.g., DePaolo, 1981; Plank and Langmuir, 1993; Tatsumi and Eggins, 1995). In particular, assimilation–fractional crystallization (AFC), in which crustal assimilation and fractional crystallization proceed simultaneously, can modify both the trace-element and isotopic compositions of magmas (e.g., DePaolo, 1981). Therefore, in order to understand magma genesis and evolutionary processes, it is important to constrain not only mantle- and slab-derived components but also the geochemical characteristics of crustal materials that may have contributed to magmas.
Southwest Japan is characterized by the zonal distribution of various geologic belts with different origins and formation histories, including the Sanbagawa metamorphic belt, the Kurosegawa tectonic belt, the Shimanto belt, and the Ryoke metamorphic rocks (e.g., Wallis et al., 2020). These basement rocks may represent important crustal end-member components for evaluating crustal contamination and assimilation processes in magmas of the Southwest Japan arc. However, there have been only limited reports of systematic whole-rock major- and trace-element compositions, together with Sr–Nd–Pb isotopic ratios, obtained from the same samples of basement rocks in Southwest Japan, which has hindered quantitative evaluation of the influence of crustal components. In this study, we aim to acquire and compile a comprehensive geochemical dataset for basement rocks in Southwest Japan, including major and trace elements and, ultimately, Sr–Nd–Pb isotopic ratios, in order to provide fundamental information on crustal end-member components useful for magma-genesis studies. The investigated samples include Ryoke metamorphic rocks, such as granulites and gneisses, from Yashiro-jima Island, Yamaguchi Prefecture; Sanbagawa metamorphic rocks, such as amphibolites and greenschists, and associated plutonic rocks from the Uruzu area, western Ehime Prefecture; and plutonic rocks of the Kurosegawa tectonic belt from the Mikame area, western Ehime Prefecture. In this preliminary study, we compare mainly major- and trace-element compositions of mafic rocks from these geologic units.
Based on trace-element ratios involving Nb, Zr, Th, and Y, the mafic basement rocks from Yashiro-jima Island, including Ryoke metamorphic rocks and gabbroic rocks, are characterized by relatively low Nb/Y ratios relative to Zr/Y ratios and relatively high Nb/Th ratios, showing geochemical features similar to arc-related mafic rocks. In contrast, some metamorphic and plutonic rocks from the Uruzu and Mikame areas in Ehime Prefecture are characterized by relatively high Nb/Y ratios relative to Zr/Y ratios and high Nb/Th ratios, resembling ocean island basalts or oceanic plateau basalts. This suggests that these rocks may include mafic rocks derived from plume-related oceanic basaltic protoliths. Although the effects of metamorphism and alteration need to be considered when estimating protoliths based on trace-element compositions, the geochemical features recognized in this study are consistent with previous studies that suggested a relationship between Ryoke mafic rocks and Cretaceous arc magmatism, as well as the occurrence of mafic rocks derived from oceanic basalts in the Sanbagawa and Kurosegawa belts (e.g., Nakajima et al., 2004; Utsunomiya et al., 2011; Osanai et al., 2014). In this presentation, we further examine the geochemical diversity of basement rocks as potential crustal end-member components by comparing our data with previously reported data for basement rocks in Southwest Japan and with xenoliths from the Hime-shima volcanic group reported by Hirayama et al. (2022). These comparisons may provide fundamental information for evaluating differences in crustal components that could have contributed to arc magmas.
Southwest Japan is characterized by the zonal distribution of various geologic belts with different origins and formation histories, including the Sanbagawa metamorphic belt, the Kurosegawa tectonic belt, the Shimanto belt, and the Ryoke metamorphic rocks (e.g., Wallis et al., 2020). These basement rocks may represent important crustal end-member components for evaluating crustal contamination and assimilation processes in magmas of the Southwest Japan arc. However, there have been only limited reports of systematic whole-rock major- and trace-element compositions, together with Sr–Nd–Pb isotopic ratios, obtained from the same samples of basement rocks in Southwest Japan, which has hindered quantitative evaluation of the influence of crustal components. In this study, we aim to acquire and compile a comprehensive geochemical dataset for basement rocks in Southwest Japan, including major and trace elements and, ultimately, Sr–Nd–Pb isotopic ratios, in order to provide fundamental information on crustal end-member components useful for magma-genesis studies. The investigated samples include Ryoke metamorphic rocks, such as granulites and gneisses, from Yashiro-jima Island, Yamaguchi Prefecture; Sanbagawa metamorphic rocks, such as amphibolites and greenschists, and associated plutonic rocks from the Uruzu area, western Ehime Prefecture; and plutonic rocks of the Kurosegawa tectonic belt from the Mikame area, western Ehime Prefecture. In this preliminary study, we compare mainly major- and trace-element compositions of mafic rocks from these geologic units.
Based on trace-element ratios involving Nb, Zr, Th, and Y, the mafic basement rocks from Yashiro-jima Island, including Ryoke metamorphic rocks and gabbroic rocks, are characterized by relatively low Nb/Y ratios relative to Zr/Y ratios and relatively high Nb/Th ratios, showing geochemical features similar to arc-related mafic rocks. In contrast, some metamorphic and plutonic rocks from the Uruzu and Mikame areas in Ehime Prefecture are characterized by relatively high Nb/Y ratios relative to Zr/Y ratios and high Nb/Th ratios, resembling ocean island basalts or oceanic plateau basalts. This suggests that these rocks may include mafic rocks derived from plume-related oceanic basaltic protoliths. Although the effects of metamorphism and alteration need to be considered when estimating protoliths based on trace-element compositions, the geochemical features recognized in this study are consistent with previous studies that suggested a relationship between Ryoke mafic rocks and Cretaceous arc magmatism, as well as the occurrence of mafic rocks derived from oceanic basalts in the Sanbagawa and Kurosegawa belts (e.g., Nakajima et al., 2004; Utsunomiya et al., 2011; Osanai et al., 2014). In this presentation, we further examine the geochemical diversity of basement rocks as potential crustal end-member components by comparing our data with previously reported data for basement rocks in Southwest Japan and with xenoliths from the Hime-shima volcanic group reported by Hirayama et al. (2022). These comparisons may provide fundamental information for evaluating differences in crustal components that could have contributed to arc magmas.