11:00 〜 11:15
[SMP22-08] Late Proterozoic to Early Paleozoic igneous activities in the Gondwana Suture Zone, Sør Rondane Mountains, East Antarctica
キーワード：南極、ゴンドワナ縫合帯、セール・ロンダネーネ山地、火成活動
The formation of the Gondwana supercontinent resulted from the collision of the East- and West-Gondwana continents. The collisional event occurred as geological time scale from the late Neoproterozoic to early Cambrian, referred as Pan-African orogen. The geology of Sør Rondane Mountains is divided into the northeast terrane and the southwest terrane separated by Main Tectonic Boundary (Osanai et al., 2013), at which the northeast terrane thrust over the southwest terrane. These two terranes were collided in the late Proterozoic (650 to 600 Ma). The igneous activities in this region occurred during syn- to post-collisional event with the long duration of magmatism from 650 Ma to 500 Ma. Elburg et al. (2016) revealed that the protracted igneous activities could be divided into four thermal pulses: at 650-600 Ma, 570-550 Ma, ca. 530 Ma, and 510-500 Ma. The 510-500 Ma pulse is recognized as a magmatic tail. Regional tectonic setting of Sør Rondane indicates compressional field before 600 Ma, then changing to extensional field (Toyoshima et al., 2013). Pulses in the 650-600 Ma range are associated with syn-collisional igneous activity, while other pulses are recognized as post-collisional ones. It is expected that the syn- to post-collisional igneous activities should reflect the tectonic background from compressional to extensional fields, and would provide clarification of magmatic processes with the transition of source region. In this presentation, we perform the geochronological and geochemical studies with some compiled data on the syn- to post-collisional igneous rocks to identify the magmatic processes of each pulse.
The 650-600 Ma rocks composed of diorite and granite, and have strong to weak foliations, while the rocks younger than 570 Ma consist of various kind of intrusive rocks including tonalite, granite, monzonite, and syenite. The post-collisional rocks show less foliated except for the rocks with localized mylonitic deformation. The 650-600 Ma rocks geochemically show metaluminous and magnesian affinities. The whole-rock epsilon Nd initial values (eNdi) are of positive from 1.2 to 3.2. The 570-550 Ma rocks except for syenite mostly show metaluminous and high-K granite similar to A-type granite. The eNdi values range from -2.6 to +0.6 mostly have negative values. These values are slightly lower, although they overlap with the range of syenite (eNdi: -1.5 to +1.0) occurring at the same pulse. The 530 Ma rocks include tonalite, granite, and monzonite, and geochemically have various compositions. The tonalite possesses metaluminous and magnesian affinities similar to the 650-600 Ma rocks, but are negative eNdi values from -0.5 to -0.3. The monzonite and granite are characterized by high-K and high-Fe contents, and A-type granite affinity. Their eNdi values are -0.7 to -0.6.
The geochemical features reveal that the 650-600 Ma syn-collisional rocks are thought to be derived from remnants of subduction related materials originally produced by the melting of metasomatized lithosphere. The post-collisional rocks show A-type granite affinity except for the 530 Ma tonalite. The post-collisional granite and monzonite can be explained as reflecting both crustal reworking and input of high-K mafic magma, such as syenite because they have high-K contents and their eNdi values show lower than 1.0. The 530 Ma tonalite with negative eNdi values would be produced by partial melting of low-K protolith probably derived from the middle Proterozoic tonalite situated in the southwest terrane. The middle Proterozoic tonalite shows metaluminous and low-K2O contents and their eNd values corrected with 530 Ma are almost identical with those of the 530 tonalite. Considering the regional tectonic setting at that time, the post-collisional igneous activities would, therefore, occurr during the extensional setting involving upwelling of the asthenosphere.
The 650-600 Ma rocks composed of diorite and granite, and have strong to weak foliations, while the rocks younger than 570 Ma consist of various kind of intrusive rocks including tonalite, granite, monzonite, and syenite. The post-collisional rocks show less foliated except for the rocks with localized mylonitic deformation. The 650-600 Ma rocks geochemically show metaluminous and magnesian affinities. The whole-rock epsilon Nd initial values (eNdi) are of positive from 1.2 to 3.2. The 570-550 Ma rocks except for syenite mostly show metaluminous and high-K granite similar to A-type granite. The eNdi values range from -2.6 to +0.6 mostly have negative values. These values are slightly lower, although they overlap with the range of syenite (eNdi: -1.5 to +1.0) occurring at the same pulse. The 530 Ma rocks include tonalite, granite, and monzonite, and geochemically have various compositions. The tonalite possesses metaluminous and magnesian affinities similar to the 650-600 Ma rocks, but are negative eNdi values from -0.5 to -0.3. The monzonite and granite are characterized by high-K and high-Fe contents, and A-type granite affinity. Their eNdi values are -0.7 to -0.6.
The geochemical features reveal that the 650-600 Ma syn-collisional rocks are thought to be derived from remnants of subduction related materials originally produced by the melting of metasomatized lithosphere. The post-collisional rocks show A-type granite affinity except for the 530 Ma tonalite. The post-collisional granite and monzonite can be explained as reflecting both crustal reworking and input of high-K mafic magma, such as syenite because they have high-K contents and their eNdi values show lower than 1.0. The 530 Ma tonalite with negative eNdi values would be produced by partial melting of low-K protolith probably derived from the middle Proterozoic tonalite situated in the southwest terrane. The middle Proterozoic tonalite shows metaluminous and low-K2O contents and their eNd values corrected with 530 Ma are almost identical with those of the 530 tonalite. Considering the regional tectonic setting at that time, the post-collisional igneous activities would, therefore, occurr during the extensional setting involving upwelling of the asthenosphere.