Solidification pressures and ages recorded in mafic microgranular enclaves and their host granite: An example of the world's youngest Kurobegawa granite

Abstract

Solidification pressures and ages of mafic microgranular enclaves (MMEs) and their host granite were determined and compared based on Al‐in‐hornblende geobarometry and U–Pb zircon dating in two sample localities in the Kurobegawa Granite. In sample KRG19‐A03 from the middle unit of the pluton, the MME and the host granite yielded 0.18 ± 0.03 to 0.24 ± 0.04 GPa and 0.16 ± 0.03 to 0.23 ± 0.04 GPa, respectively. The MME and the host granite of sample KRG19‐B08b from the lower unit, respectively, yielded 0.12 ± 0.02 to 0.21 ± 0.03 GPa and 0.13 ± 0.02 to 0.18 ± 0.03 GPa. In each sample locality, the estimated solidification pressures of the MME and its host granite overlap. The weighted mean ages were calculated as 0.775 ± 0.045 Ma and 0.831 ± 0.055 Ma for the MME and the host granite of KRG19‐A03, respectively. The MME and the host granite of KRG19‐B08b, respectively, yielded 0.672 ± 0.033 Ma and 0.735 ± 0.042 Ma. The ages for MMEs tend to be younger than the host granites, although they overlap within uncertainty. Zircon commonly occurs as the matrix minerals in both lithologies, meanwhile, zircon also occurs as early phases in plagioclase cores only in the host granites. Such differences in mode of occurrence of zircon suggest that the age variation reflects the differences in timing of zircon crystallization between the lithologies. Therefore, the MMEs record the same solidification pressures as the host granites and better represent the final solidification timing of the pluton. From these data of the MMEs, an average exhumation rate of each sample locality was estimated as 7.1–14.5 mm/year (KRG19‐A03) and 5.5–14.4 mm/year (KRG19‐B08b). These exhumation rates are much larger than that of the ca. 5.6–5.2 Ma Shiaidani Granodiorite (0.93–2.5 mm/year), implying that drastic change of the exhumation rate took place between ca. 5.2 Ma and ca. 0.83 Ma.

Short communication: Inverse correlation between radiation damage and fission-track etching time on monazite

“…KRG19-B04 36.6430 137.6860 Kurobegawa Granodiorite lower unit (Harayama et al, 2000) biotite-hornblende granodiorite ca. 0.8 Ma (ZUPb: Ito et al, 2013;Suzuki et al, 2022) < 1 Ma (ZFT and ZHe: Yamada, 1999;King et al, 2023) Table 2. Representive results of Raman and electron probe micro analyses, calculated Δ FWHM , and t MFT for monazites in 5 rock samples.…”

“…Zircon U-Pb ages indicate that this is the youngest plutonic rock presently exposed on the surface of the earth (ca. 0.8 Ma; Ito et al, 2013, 2017, Suzuki et al, 2022. This granodiorite is characterized by numerous mafic magmatic enclaves and is subdivided into three units-lower, middle, and upper-based on the areal proportion of mafic magmatic enclaves in an outcrop (Harayama et al, 2000).…”

Magnetic susceptibility, mineral chemistry, and geothermobarometry of granitoids from Lohit Plutonic Complex, Arunachal Trans‐Himalaya, Northeast India: Implications on emplacement and crystallization conditions of oxidized calc alkaline magmatic arc system

“…However, most of the amphiboles from quartz diorite and granodiorite bear lower Fe# (0.32-0.59) as compared to the leucogranite (0.58-0.61) of the LPC, which correlate very well with the moderate to high oxidizing conditions (Table S2; Figure 9a). Moreover, the magnesio-hornblende rim may have been crystallized under a progressively oxidizing condition during the second boiling of residual melt of host magmas and the transformation of sulfur species from SO 2 to H 2 S within the microgranular enclaves on cooling, as opined elsewhere (Suzuki et al, 2022;Yamaguchi et al, 2003). However, this aspect needs to be investigated further by conducting a detailed investigation on the mafic to hybrid microgranular enclaves and host LPC granitoids.…”