5:15 PM - 7:15 PM
[SCG57-P06] Investigation of inherited zircons in felsic igneous rocks of the Setouchi Volcanic Rocks
Keywords:U-Pb age, LA-ICPMS, zircon, Setouchi Volcanic Rocks, sediment
The ages of inherited zircons in igneous rocks provide valuable insights into their magma sources. For example, the detrital sources of sediments contributing to S-type granites have been inferred from multiple age populations [1], while the contribution of subducted sediments to arc andesites and dacites has been estimated based on the ages of inherited zircons [2].
The Setouchi Volcanic Rocks were active mostly between 15–14 Ma in the trench-proximal regions of the southwest Japan arc during the Middle Miocene. While the occurrence of high-Mg andesites, which are considered to have equilibrated with mantle peridotite, has drawn significant attention, a substantial amount of felsic volcanic rocks was also present. Among the Setouchi felsic volcanic rocks in the Kii Peninsula, some lithologies exhibit depletion in Y and heavy rare earth elements (HREE) with little to no Eu negative anomaly. Their high-K and peraluminous whole-rock compositions suggest an origin involving the melting of sediments at depths where plagioclase is scarce and garnet is a residual phase [3]. Similarly, rhyolites from Shodoshima Island in the Seto Inland Sea, which have comparable chemical compositions to those in Kii penisula, are inferred to have originated from the melting of subducted terrigenous sediments on the Shikoku Basin slab, based on Pb isotope similarities and comparison with experimental melt compositions [4].
To further investigate the source of these felsic volcanic rocks, which have been hypothesized to result from sediment melting at mantle depths, we analyzed the zircon U-Pb ages. Zircons were obtained from two samples of the Donzurubo Formation of the Nijo Group, which is part of the Setouchi Volcanic Rocks in the Kii Peninsula and predominantly produces HREE-depleted felsic volcanic rocks. The U-Pb dating analyses were conducted using a femtosecond laser ablation system coupled with a magnetic-sector ICP mass spectrometer at the Geochemical Research Center, University of Tokyo [5]. Cathodoluminescence (CL) imaging was used to select homogeneous areas for analysis, yielding mostly concordant U-Pb ages. Most zircon grains corresponded to an igneous crystallization age of approximately 15 Ma, but inherited zircons with older ages were also identified. Their 238U-206Pb ages ranged from 35 to 1830 Ma, with the highest frequency observed in the 61–78 Ma range. The granitic rocks of the Ryoke and Sanyo belts in the Kinki region are dated between 100 and 75 Ma [6]. The local basement rock, though not directly dated, is likely part of the Phase 1 Ryoke Granites, as inferred from their intrusive relationships. Therefore, the inherited zircon grains are likely younger than the direct basement rock. However, zircons with ages between 77 and 65 Ma have been reported from the Ao Granite, which corresponds to Phase 4 Ryoke Granites [7]. It is possible that younger granitic bodies are present beneath the region and have contributed to the inherited zircon population. Additionally, some grains yielded even younger ages of 59, 51, and 35 Ma. These zircons may have been derived from sediments carried to the depth by the subduction of the Shikoku Basin slab or deeply accreted sediments from underplating processes [8].
[1] Mass et al. (2001) J. Pet., 42, 1429. [2] Gómez-Tuena et al. (2018) Lithos, 322, 52. [3] Shinjoe et al. (2007) J. Geol. Soc. Jpn, 113, 310. [4] Shimoda & Tatsumi (1999) Isl. Arc, 8, 383. [5] Hirata & Iwano (2024) In Methods and Applications of Geochronology (Elsevier), 105. [6] Nakajima (2017) J. Geol. Soc. Jpn, 124, 603. [7] Higashino et al. (2023) 2023 annual meeting of JAMS, R6-07. [8] Isozaki et al. (2010) Gondwana Res., 18, 82.
The Setouchi Volcanic Rocks were active mostly between 15–14 Ma in the trench-proximal regions of the southwest Japan arc during the Middle Miocene. While the occurrence of high-Mg andesites, which are considered to have equilibrated with mantle peridotite, has drawn significant attention, a substantial amount of felsic volcanic rocks was also present. Among the Setouchi felsic volcanic rocks in the Kii Peninsula, some lithologies exhibit depletion in Y and heavy rare earth elements (HREE) with little to no Eu negative anomaly. Their high-K and peraluminous whole-rock compositions suggest an origin involving the melting of sediments at depths where plagioclase is scarce and garnet is a residual phase [3]. Similarly, rhyolites from Shodoshima Island in the Seto Inland Sea, which have comparable chemical compositions to those in Kii penisula, are inferred to have originated from the melting of subducted terrigenous sediments on the Shikoku Basin slab, based on Pb isotope similarities and comparison with experimental melt compositions [4].
To further investigate the source of these felsic volcanic rocks, which have been hypothesized to result from sediment melting at mantle depths, we analyzed the zircon U-Pb ages. Zircons were obtained from two samples of the Donzurubo Formation of the Nijo Group, which is part of the Setouchi Volcanic Rocks in the Kii Peninsula and predominantly produces HREE-depleted felsic volcanic rocks. The U-Pb dating analyses were conducted using a femtosecond laser ablation system coupled with a magnetic-sector ICP mass spectrometer at the Geochemical Research Center, University of Tokyo [5]. Cathodoluminescence (CL) imaging was used to select homogeneous areas for analysis, yielding mostly concordant U-Pb ages. Most zircon grains corresponded to an igneous crystallization age of approximately 15 Ma, but inherited zircons with older ages were also identified. Their 238U-206Pb ages ranged from 35 to 1830 Ma, with the highest frequency observed in the 61–78 Ma range. The granitic rocks of the Ryoke and Sanyo belts in the Kinki region are dated between 100 and 75 Ma [6]. The local basement rock, though not directly dated, is likely part of the Phase 1 Ryoke Granites, as inferred from their intrusive relationships. Therefore, the inherited zircon grains are likely younger than the direct basement rock. However, zircons with ages between 77 and 65 Ma have been reported from the Ao Granite, which corresponds to Phase 4 Ryoke Granites [7]. It is possible that younger granitic bodies are present beneath the region and have contributed to the inherited zircon population. Additionally, some grains yielded even younger ages of 59, 51, and 35 Ma. These zircons may have been derived from sediments carried to the depth by the subduction of the Shikoku Basin slab or deeply accreted sediments from underplating processes [8].
[1] Mass et al. (2001) J. Pet., 42, 1429. [2] Gómez-Tuena et al. (2018) Lithos, 322, 52. [3] Shinjoe et al. (2007) J. Geol. Soc. Jpn, 113, 310. [4] Shimoda & Tatsumi (1999) Isl. Arc, 8, 383. [5] Hirata & Iwano (2024) In Methods and Applications of Geochronology (Elsevier), 105. [6] Nakajima (2017) J. Geol. Soc. Jpn, 124, 603. [7] Higashino et al. (2023) 2023 annual meeting of JAMS, R6-07. [8] Isozaki et al. (2010) Gondwana Res., 18, 82.