5:15 PM - 7:15 PM
[SMP26-P02] The occurrences of epidote and contrastive P–T paths of intermediate to mafic gneisses from Akebono Rock in the Lützow-Holm Complex, East Antarctica
Keywords:intermediate to mafic gneisses, contrastive pressure–temperature paths, Akebono Rock, Lützow-Holm Complex, East Antarctica
The Lützow-Holm Complex (LHC) in East Antarctica consists mainly of Ediacaran to Cambrian metamorphic rocks formed during the Gondwana amalgamation and is divided traditionally into amphibolite-facies, transitional and granulite-facies zones from northeast to southwest (e.g., Hiroi et al., 1983; Shiraishi et al., 1994). Minor Tonian metamorphic rocks with amphibolite- to granulite-facies grade have been also identified from the amphibolite-facies zone in the northeast LHC. Akebono Rock is one of the outcrops and composed of amphibolite-facies metamorphosed rocks at ca. 940–930 Ma (Baba et al., 2022; Kitano et al., 2023), such as biotite gneiss, garnet–biotite gneiss, biotite–hornblende gneiss, calc-silicate rock and amphibolite intruded by granite, pegmatite and slightly recrystallized basaltic to andesitic dyke (Hiroi et al., 1986). Recently, Baba et al. (2020, 2022) revealed the deformation and magmatic history in Akebono Rock and clockwise pressure (P)–temperature (T) paths with the peak condition of ca. 6–8 kbar and 700 ℃ at ca. 940 Ma from pelitic gneisses and amphibolites in western Akebono Rock. On the other hand, Kitano et al. (2024) reported an anti-clockwise P–T path with the peak condition of ca. 5–8 kbar and 590–660 ℃ from a staurolite-bearing mafic gneiss, of which metamorphic age at ca. 930 Ma was obtained by zircon U–Pb dating (Kitano et al., 2023), in eastern Akebono Rock. Thus, the understanding of their special relation between rocks showing the clockwise and anti-clockwise P–T paths is a key issue to decipher the metamorphic evolution in this area. This study provides petrographic features of intermediate to mafic gneisses and contrastive P–T paths of two garnet–hornblende–biotite gneisses.
Those gneisses in Akebono Rock occur as interlayers alternating with felsic gneisses or blocks within them. Petrographically, we identified two modes of occurrences of epidote. One is the euhedral to subhedral and fine- to medium-grain in the matrix replacing hornblende as a secondary phase occasionally coexisting with garnet. The other is the subhedral to anhedral fine-grained inclusion in plagioclase or garnet as a relic phase with or without allanite. The former and latter are common in central to eastern and western Akebono Rock, respectively. The petrological analysis for the garnet-bearing gneiss from the central part indicated subhedral to anhedral garnet of which Ca contents increase toward rim (core, Alm61Pyr8Sps11Grs20; mantle, Alm58Pyr6Sps11Grs24; rim, Alm56Pyr5Sps9Grs30) with inclusions of biotite + plagioclase + quartz ± epidote ± ilmenite ± titanite and minor occurrences of epidote as inclusions in high-Ca garnet rim (XPs = 0.20–0.22) or secondary phases replacing hornblende in the matrix (XPs = 0.19–0.20). On the other hand, the analysis for the gneiss from the western part indicated subhedral oscillatory zoned garnet of which Ca contents gradually change (core, Alm59Pyr10Sps8Grs23; mantle, Alm54Pyr9Sps7Grs30; inner rim, Alm61Pyr10Sps7Grs22; outer rim, Alm56Pyr7Sps8Grs28) with inclusions of hornblende + biotite + plagioclase ± quartz ± K-feldspar ± muscovite ± albite ± epidote ± ilmenite ± titanite and relic epidote included by plagioclase with allanite core (XPs = 0.12–0.19) or garnet mantle to inner rim (XPs = 0.11–0.18). Applying to geothermobarometries and petrogenetic grids for inclusions in garnet porphyroblasts and matrix phases yielded the anti-clockwise P–T path for the gneiss from the central part and the clockwise one for that from the western part. The contrastive P–T paths deduced by this study and previous studies possibly indicate different tectono-thermal histories between the central to eastern and western Akebono Rock and support Baba et al. (2020) who identified the discrepancies of deformation and magmatic histories between the eastern and western regions. The tectonic implications will be discussed in the presentation.
Those gneisses in Akebono Rock occur as interlayers alternating with felsic gneisses or blocks within them. Petrographically, we identified two modes of occurrences of epidote. One is the euhedral to subhedral and fine- to medium-grain in the matrix replacing hornblende as a secondary phase occasionally coexisting with garnet. The other is the subhedral to anhedral fine-grained inclusion in plagioclase or garnet as a relic phase with or without allanite. The former and latter are common in central to eastern and western Akebono Rock, respectively. The petrological analysis for the garnet-bearing gneiss from the central part indicated subhedral to anhedral garnet of which Ca contents increase toward rim (core, Alm61Pyr8Sps11Grs20; mantle, Alm58Pyr6Sps11Grs24; rim, Alm56Pyr5Sps9Grs30) with inclusions of biotite + plagioclase + quartz ± epidote ± ilmenite ± titanite and minor occurrences of epidote as inclusions in high-Ca garnet rim (XPs = 0.20–0.22) or secondary phases replacing hornblende in the matrix (XPs = 0.19–0.20). On the other hand, the analysis for the gneiss from the western part indicated subhedral oscillatory zoned garnet of which Ca contents gradually change (core, Alm59Pyr10Sps8Grs23; mantle, Alm54Pyr9Sps7Grs30; inner rim, Alm61Pyr10Sps7Grs22; outer rim, Alm56Pyr7Sps8Grs28) with inclusions of hornblende + biotite + plagioclase ± quartz ± K-feldspar ± muscovite ± albite ± epidote ± ilmenite ± titanite and relic epidote included by plagioclase with allanite core (XPs = 0.12–0.19) or garnet mantle to inner rim (XPs = 0.11–0.18). Applying to geothermobarometries and petrogenetic grids for inclusions in garnet porphyroblasts and matrix phases yielded the anti-clockwise P–T path for the gneiss from the central part and the clockwise one for that from the western part. The contrastive P–T paths deduced by this study and previous studies possibly indicate different tectono-thermal histories between the central to eastern and western Akebono Rock and support Baba et al. (2020) who identified the discrepancies of deformation and magmatic histories between the eastern and western regions. The tectonic implications will be discussed in the presentation.