14:15 〜 14:30
[SMP25-03] Average cooling rate of a Neoproterozoic-Cambrian UHT terrane (Rundvågshetta, Lützow-Holm Complex, East Antarctica)
キーワード：超高温変成作用、ジルコン、冷却速度
Estimating cooling rate of ultrahigh-temperature (UHT) granulites in continental collision zones is essential to understand their tectonic processes [e.g., 1]. In the case of Rundvågshetta (Lützow-Holm Complex, East Antarctica), which is a Neoproterozoic-Cambrian UHT terrane, [2] estimated average cooling rate of ~ 30 oC/Myr by using the zircon U-Pb age (ca. 520 Ma) from a syndeformational leucosome and biotite K/Ar age (ca. 500 Ma/~ 300 oC). They assumed that the zircon age represents the timing of peak metamorphism (~ 900 oC/~ 11 kbar). However, zircon U-Pb ages do not always represent the timing of peak metamorphic conditions [e.g., 3]. Additionally, it is recently suggested that UHT granulites in Rundvågshetta experienced rapid cooling based on the occurrence of felsite inclusions (FIs) in garnet [4]. [5] suggested that the cooling rate 1 to 2 orders of magnitude faster than the previous estimate for regionally metamorphosed granulite masses is required to explain the preservation of Ti zoning in quartz and dendritic microtextures in the FIs. However, because melt inclusions with quartz + feldspar micrographic intergrowths similar to FIs [5] are also reported from slowly cooled (~ 8 oC/Myr; [6]) UHT granulites in the Kerala Khondalite Belt, southern India [7], whether rapid cooling is necessary for the preservation of the dendritic microtextures in FIs is still unclear. Therefore, in this study, we reevaluate the cooling rate of an UHT granulite (sample TK2003010309) from Rundvågshetta by applying petrochronological approach.
Garnet in the studied sample has P-poor core, P-rich mantle and P-poor rim. Based on the detailed petrography of inclusions in garnet, [8] interpreted that the garnet core, mantle and rim were respectively formed in the prograde, peak and retrograde stages along a clockwise pressure-temperature evolution. Because the garnet rim is not replaced by orthopyroxene + cordierite symplectite, [8] interpreted that the garnet rim was formed after the post-peak isothermal decompression.
Zircon in the rock matrix and included in the garnet rim commonly has metamorphic rim with variable Th/U ratio of 0.08-1.13 [9]. The zircon rim includes sillimanite, K-feldspar and rutile and shows weighted mean age of 530.5 ± 4.9 Ma (2σ error, n = 13, MSWD = 1.5; [10]). Applying the Ti-in-zircon geothermometer [11] to the zircon rim gave 777-820 oC. Array plot analysis [12] of REE partitioning between the garnet and the zircon revealed that the zircon rim was formed in equilibrium with the garnet rim. Therefore, the U-Pb age of zircon rim (ca. 530 Ma) represents the timing of retrograde stage after the post-peak isothermal decompression, and thus the average cooling rate needs to be re-examined. By combining our zircon data with the biotite K/Ar age of [2], the average cooling rate after the post-peak isothermal decompression is calculated as ~ 13-20 oC/Myr. Therefore, if we assume the constant cooling rate from ~ 800 oC to ~ 300 oC, the 1 to 2 orders of magnitude rapid cooling suggested by [4] is not likely. Further studies using geochronometers with closure temperatures between ~ 800 oC to ~ 300 oC (e.g., rutile U-Pb dating; [13]) is required to test the rapid cooling model.
References
[1] Harley (2016) JMPS. [2] Fraser et al. (2000) JMG. [3] Harley et al. (2007) Elements. [4] Hiroi et al. (2019) JMPS. [5] Hiroi et al. (2020) Island Arc. [6] Ferrero & Angel (2018) J. Petrol. [7] Cesare et al. (2011) J. Virtual Explorer. [8] Suzuki & Kawakami (2020) Abstract of Annual Meeting of JAMS. [9] Suzuki et al. (2021) Abstract of the 12th Symposium on Polar Science. [10] Suzuki et al. (2022) Abstract of EGU General Assembly 2022. [11] Ferry & Watson (2007) CMP. [12] Taylor et al. (2017) JMG. [13] Johnson & Harley (2012) CUP.
Garnet in the studied sample has P-poor core, P-rich mantle and P-poor rim. Based on the detailed petrography of inclusions in garnet, [8] interpreted that the garnet core, mantle and rim were respectively formed in the prograde, peak and retrograde stages along a clockwise pressure-temperature evolution. Because the garnet rim is not replaced by orthopyroxene + cordierite symplectite, [8] interpreted that the garnet rim was formed after the post-peak isothermal decompression.
Zircon in the rock matrix and included in the garnet rim commonly has metamorphic rim with variable Th/U ratio of 0.08-1.13 [9]. The zircon rim includes sillimanite, K-feldspar and rutile and shows weighted mean age of 530.5 ± 4.9 Ma (2σ error, n = 13, MSWD = 1.5; [10]). Applying the Ti-in-zircon geothermometer [11] to the zircon rim gave 777-820 oC. Array plot analysis [12] of REE partitioning between the garnet and the zircon revealed that the zircon rim was formed in equilibrium with the garnet rim. Therefore, the U-Pb age of zircon rim (ca. 530 Ma) represents the timing of retrograde stage after the post-peak isothermal decompression, and thus the average cooling rate needs to be re-examined. By combining our zircon data with the biotite K/Ar age of [2], the average cooling rate after the post-peak isothermal decompression is calculated as ~ 13-20 oC/Myr. Therefore, if we assume the constant cooling rate from ~ 800 oC to ~ 300 oC, the 1 to 2 orders of magnitude rapid cooling suggested by [4] is not likely. Further studies using geochronometers with closure temperatures between ~ 800 oC to ~ 300 oC (e.g., rutile U-Pb dating; [13]) is required to test the rapid cooling model.
References
[1] Harley (2016) JMPS. [2] Fraser et al. (2000) JMG. [3] Harley et al. (2007) Elements. [4] Hiroi et al. (2019) JMPS. [5] Hiroi et al. (2020) Island Arc. [6] Ferrero & Angel (2018) J. Petrol. [7] Cesare et al. (2011) J. Virtual Explorer. [8] Suzuki & Kawakami (2020) Abstract of Annual Meeting of JAMS. [9] Suzuki et al. (2021) Abstract of the 12th Symposium on Polar Science. [10] Suzuki et al. (2022) Abstract of EGU General Assembly 2022. [11] Ferry & Watson (2007) CMP. [12] Taylor et al. (2017) JMG. [13] Johnson & Harley (2012) CUP.