5:15 PM - 7:15 PM
[SMP26-P03] High-grade metamorphic rocks in Instekleppane, Lützow-Holm Complex, East Antarctica: A potential new occurrence of ultrahigh-temperature (UHT) metamorphic rocks
Keywords:Ultrahigh-temperature (UHT) metamorphic rocks, Lützow-Holm Complex, East Antarctica
Eastern part of the Dronning Maud Land including the Lützow-Holm Complex, East Antarctica is thought to have been situated in the collision zone between East and West Gondwana during the final stage of amalgamation of the Gondwana supercontinent (e.g., Stern, 1994, Ann. Rev. Earth Planetary Sci.). Several orogenies explaining the amalgamation of the Gondwana have been discussed: Jacobs and Thomas (2004, Geology) proposed East African-Antarctic Orogen (EAAO) which is one huge orogenic belt that was active for a long period (ca. 650-500 Ma), on the other hand, Meert (2003, Tectonophysics) proposed two crossing orogens where East African Orogen (EAO, Stern, 1994) being active during 750-620 Ma and Kuunga Orogen during 580-500 Ma, however there is no consensus about geological history of the orogenies so far.
The main tectonothermal event of the Lützow-Holm Complex was previously thought to have occurred around 550-530 Ma. However a summary of U-Pb ages from this complex (Dunkley et al., 2020, Polar Sci.) suggests that ca. 600 Ma rocks are sporadically distributed in the southern part of the Soya Coast. In such context, the 65th Japan Antarctic Research Expedition (JARE65) conducted a geological survey one of its targets being to reveal the distribution of ca. 600 Ma metamorphic/igneous rocks in the LHC. In this study, we present a preliminary report on the geology of Instekleppane, where the first multi-day geological survey was conducted by JARE65.
Instekleppane is an exposure approximately 1.5 km × 2.0 km, located in the southernmost part of the Lützow-Holm Bay, on the eastern shore of the Shirase Glacier. The common lithologies of this exposure are felsic Opx-Bt gneiss and leucocratic Grt-Bt gneiss with mafic to ultramafic Opx-Cpx-Hbl granulites, felsic Grt-Opx-Bt gneisses and Mg-Al rocks such as Grt-Crd-Bt gneiss and Spr-Crd-Bt gneiss intercalated as layer and lenses. Grt-bearing rocks are distinctive at the western and the southern parts of this exposure.
Spr-Crd-Bt gneiss (TA2024012401A) in the southern part of Instekleppane contains Bt, Crd, Spr and Spl in the matrix with relic Grt, Sil and Opx surrounded by Crd. Qz and Fls are not recognized in the matrix or as inclusions in Grt. Sil and Opx are found as inclusion in Grt. Crd shows intergrowth with Spr ± Spl in the matrix. These observations indicate a change in metamorphic conditions from Grt+Sil+Opx stable condition to Crd+Spr±Spl stable condition, i.e., decompression under P-T conditions higher than 950 °C and 0.8 GPa (Harley, 2004, JMPS), indicating ultrahigh temperature (UHT) metamorphism. Such change in metamorphic conditions are similar to those reported from several localities in the LHC (Rundvågshetta, Yoshimura et al., 2008, GSL Spec. Publ.; Skallevikshalsen, Kawasaki et al., 2013, GSL Spec. Publ.). Grt-Crd-Opx-Spr gneiss (TA2024012304E) also contains Crd+Spr sympelctite after Opx+Sil as well as Opx+Spr symplectite replacing Grt.
Such a texuture implying UHT condition has been also found in the western part. Grt-Opx-Crd gneiss (TA2024012701C) contains Spr inclusion in Grt and Qz is abundant in the matrix. This possibly suggests that Spr+Qz coexisted during a certain metamorphic stage. However, the felsic Grt-Opx gneiss (TA2024012604) distributed at the northern area of the western part is estimated to have formed under conditions of approximately 800°C and 0.5 GPa, based on the chemical composition of its constituent minerals. Compared to the previously mentioned rocks, it clearly indicates lower temperature and pressure conditions, suggesting that there may be a gap in metamorphic conditions within Instekleppane.
In the future, we will analyze the P-T-t paths of metamorphic rocks distributed in Instekleppane, particularly focused on differences between Grt-bearing and -free rocks, and also compare P-T-t paths of rocks in Instekleppane the with the neibouring exposures in order to discuss geological evolution of LHC.
The main tectonothermal event of the Lützow-Holm Complex was previously thought to have occurred around 550-530 Ma. However a summary of U-Pb ages from this complex (Dunkley et al., 2020, Polar Sci.) suggests that ca. 600 Ma rocks are sporadically distributed in the southern part of the Soya Coast. In such context, the 65th Japan Antarctic Research Expedition (JARE65) conducted a geological survey one of its targets being to reveal the distribution of ca. 600 Ma metamorphic/igneous rocks in the LHC. In this study, we present a preliminary report on the geology of Instekleppane, where the first multi-day geological survey was conducted by JARE65.
Instekleppane is an exposure approximately 1.5 km × 2.0 km, located in the southernmost part of the Lützow-Holm Bay, on the eastern shore of the Shirase Glacier. The common lithologies of this exposure are felsic Opx-Bt gneiss and leucocratic Grt-Bt gneiss with mafic to ultramafic Opx-Cpx-Hbl granulites, felsic Grt-Opx-Bt gneisses and Mg-Al rocks such as Grt-Crd-Bt gneiss and Spr-Crd-Bt gneiss intercalated as layer and lenses. Grt-bearing rocks are distinctive at the western and the southern parts of this exposure.
Spr-Crd-Bt gneiss (TA2024012401A) in the southern part of Instekleppane contains Bt, Crd, Spr and Spl in the matrix with relic Grt, Sil and Opx surrounded by Crd. Qz and Fls are not recognized in the matrix or as inclusions in Grt. Sil and Opx are found as inclusion in Grt. Crd shows intergrowth with Spr ± Spl in the matrix. These observations indicate a change in metamorphic conditions from Grt+Sil+Opx stable condition to Crd+Spr±Spl stable condition, i.e., decompression under P-T conditions higher than 950 °C and 0.8 GPa (Harley, 2004, JMPS), indicating ultrahigh temperature (UHT) metamorphism. Such change in metamorphic conditions are similar to those reported from several localities in the LHC (Rundvågshetta, Yoshimura et al., 2008, GSL Spec. Publ.; Skallevikshalsen, Kawasaki et al., 2013, GSL Spec. Publ.). Grt-Crd-Opx-Spr gneiss (TA2024012304E) also contains Crd+Spr sympelctite after Opx+Sil as well as Opx+Spr symplectite replacing Grt.
Such a texuture implying UHT condition has been also found in the western part. Grt-Opx-Crd gneiss (TA2024012701C) contains Spr inclusion in Grt and Qz is abundant in the matrix. This possibly suggests that Spr+Qz coexisted during a certain metamorphic stage. However, the felsic Grt-Opx gneiss (TA2024012604) distributed at the northern area of the western part is estimated to have formed under conditions of approximately 800°C and 0.5 GPa, based on the chemical composition of its constituent minerals. Compared to the previously mentioned rocks, it clearly indicates lower temperature and pressure conditions, suggesting that there may be a gap in metamorphic conditions within Instekleppane.
In the future, we will analyze the P-T-t paths of metamorphic rocks distributed in Instekleppane, particularly focused on differences between Grt-bearing and -free rocks, and also compare P-T-t paths of rocks in Instekleppane the with the neibouring exposures in order to discuss geological evolution of LHC.