[S-MP26] Supercontinents and Crustal Evolution

[SMP26-07] Crustal materials incorporated into the Gondwana Suture Zone: Example for post-collisional magmatic activities in the Sør Rondane Mountains, East Antarctica

*Masaaki Owada¹, Tsuyoshi Toyoshima², Atsushi Kamei³, Yasuhito Osanai⁴, Nobuhiko Nakano⁴, Tatsuro Adachi⁴, Ippei Kitano⁵ (1.Graduate School of Science and Engineering for Innovation, Yamaguchi University, 2.Niigata University , 3.Shimane University, 4.Kyushu University , 5.Hokkaido University )

Keywords:Antarctica, Gondwana suture zone, Sør Rondane Mountains, Igneous activities

The Sør Rondane Mountains (SRM) in eastern Dronning Maud Land, East Antarctica, are located within the Gondwana suture zone. The geology of the SRM is divided into the northeast terrane and the southwest terrane, which are separated by the Main Tectonic Boundary (Osanai et al., 2013). During 650–600 Ma, the northeast terrane was thrust over the southwest terrane. Igneous activity in this region occurred during syn- to post-collisional events, with a prolonged magmatic history spanning from 650 Ma to 500 Ma. Elburg et al. (2016) identified that this protracted igneous activity can be divided into three main thermal pulses: 650–600 Ma, 570–550 Ma, and approximately 530 Ma. The latter two pulses correspond to post-collisional magmatism. The presence of mafic intrusive rocks, including minette and syenite, dated to 570–550 Ma, has been interpreted as evidence of juvenile mantle input (Owada et al., 2013; Elburg et al., 2016). The granitoids dated to 530 Ma exhibit a wide range of zircon εHf values, from -5.9 to +1.3 (Elburg et al., 2016), indicating both crustal reworking and juvenile input. However, mafic intrusive rocks associated with this magmatic event have not yet been identified. Li et al. (2003) reported the presence of quartz-bearing syenite, which is classified as monzonite in the Q-A-P diagram, from the Mefjell plutonic complex, located in the central part of the SRM. This monzonite is potentially influenced by mantle-derived components. This study focuses on post-collisional magmatic pulses in the SRM over a period of 40 million years, with particular attention to the petrogenetic characteristics of the Mefjell plutonic complex associated with the 530 Ma pulse. The results of this study will provide valuable insights into post-collisional crustal evolution.

The Mefjell plutonic complex consists primarily of monzonite, granite, and tonalite. The monzonite and granite are characterized by high K and Fe contents, exhibiting an A-type granite affinity. Zircon U–Pb dating of the monzonite yields an age of 531.6 ± 5.6 Ma. In contrast, the tonalite is characterized by low K and high Sr and Al contents, resembling adakitic rocks. This indicates that both high-K and low-K magmatic rocks coexisted within the same plutonic complex at 530 Ma. The monzonite is composed of an olivine-orthopyroxene-quartz assemblage, with plagioclase, alkali feldspar, and clinopyroxene as additional constituent minerals. Biotite and amphibole, along with granular quartz, locally surround pyroxenes. Geochemically, the monzonite contains SiO₂ =54–62 wt% and K₂O = 4.3–6.8 wt%, with notably high Zr and Ba concentrations, reaching up to 2000 ppm and 5000 ppm, respectively. Zircon saturation thermometry suggests a high magmatic temperature of up to 1000°C. These petrological characteristics indicate that the monzonite shares similarities with anhydrous A-type granitic magmas, which are typically associated with high-temperature magmatism. Meanwhile, the tonalite is composed of plagioclase, quartz, clinopyroxene, orthopyroxene, amphibole, and biotite, with minor amounts of alkali feldspar. It has Al₂O₃ =17–18 wt% and a K₂O/Na₂O ratio of 0.22–0.33, resembling adakitic rocks derived from subducted oceanic slabs (Kamei et al., 2009). The monzonite and tonalite exhibit similar εNd values, ranging from -0.6 to -0.3.

Based on Nd and Hf isotopic values combined with geochemical characteristics, the magmatic activity at approximately 530 Ma in the SRM was generated through the partial melting of various crustal rocks, with the involvement of minor amounts of mantle-derived melt. This magmatic event was influenced not only by the upwelling of the asthenosphere but also by an additional heat source, possibly radiogenic heating. This is because a significant amount of crustal material containing radiogenic elements was incorporated into the suture zone during the collisional event.