11:15 〜 11:30
[SMP25-03] High-K mafic magma derived from metasomatized lithospheric mantle in the Gondwana suture zone, Eastern Dronning Maud Land, East Antarctica
キーワード：南極大陸、ゴンドワナ縫合帯、高カリウム苦鉄質マグマ
This presentation addresses post-kinematic magmatism of the Sør Rondane Mountains (SRM), Eastern Dronning Maud Land in the Gondwana suture zone, and discusses the petrogenesis and tectonic processes of high-K mafic magma. The formation of the Gondwana supercontinent resulted from the collision of the East- and West-Gondwana continents. The collisional event is regarded as geological time scale from the late Neoproterozoic to early Cambrian, Pan-African orogen.
The geology of SRM is divided into the northeast terrane and the southwest terrane. These two terranes were collided in the Late Proterozoic (c. 640 Ma), at which both terranes underwent the amphibolite to granulite facies metamorphism. After the collisional event, the tectonic setting in the SRM changed from compressional to extensional regimes. Post-kinematic intrusive rocks intruded the host rocks during the extensional setting.
The post-kinematic syenite complex in the Lunckeryggen, the central part of SRM, consists of layered syenite, melanosyenite dike and quartz syenite dike. The lamprophyre also occurs as the post-kinematic intrusive rocks. The U-Pb zircon dating of the syenite complex shows ages of 559.4 +/- 1.6 Ma and 548.8 +/- 3.4 Ma. The recalculated Pb-Pb age of the lamprophyre gives 557.5 +/- 4.8 Ma. Therefore, the syenite complex and lamprophyre intruded into this suture zone during the same magmatic stage. The syenite complex and the lamprophyre have significant character with high-K (K2O/Na2O>3) and high-(LREE/HREE) ratios (normalized La/Yb>10) with SiO2 contents ranging from 40 to 65 wt%. In addition, the syenite and lamprophyre possess the relatively enriched epsilon Nd values (+1 to -1.5) rather than the pre-existent metamorphosed intrusive rocks (1000 to 770 Ma; epsilon Nd values from +6 to +1) in the southwest terrane.
Considering petrography and geochemistry, the syenite complex was originated from the lamprophyre magma, and fractional crystallization and accumulation played an important role of formation of the layered structure. The primitive composition of the lamprophyre contains phenocrysts of Mg- and Cr-rich phlogopite in equilibrium with K-rich basaltic magma. The syenite magma emplaced at the upper crustal levels (<300 MPa) during the ascent of the crust. These features suggest that the high-K mafic magma was produced by partial melting of the lithospheric mantle that was altered by subduction and/or collisional processes. The upwelling of asthenosphere under the extensional setting would be acted as the heat source to cause the partial melting of lithosphere. As the petrogenesis of the Cambrian Yamato syenite, 300 km west of the SRM, resembles that of the syenite complex from the SRM, similar petrogenetic model may also apply to the origin of high-K mafic magma in the Gondwana suture zone during the Pan-African stage.
The geology of SRM is divided into the northeast terrane and the southwest terrane. These two terranes were collided in the Late Proterozoic (c. 640 Ma), at which both terranes underwent the amphibolite to granulite facies metamorphism. After the collisional event, the tectonic setting in the SRM changed from compressional to extensional regimes. Post-kinematic intrusive rocks intruded the host rocks during the extensional setting.
The post-kinematic syenite complex in the Lunckeryggen, the central part of SRM, consists of layered syenite, melanosyenite dike and quartz syenite dike. The lamprophyre also occurs as the post-kinematic intrusive rocks. The U-Pb zircon dating of the syenite complex shows ages of 559.4 +/- 1.6 Ma and 548.8 +/- 3.4 Ma. The recalculated Pb-Pb age of the lamprophyre gives 557.5 +/- 4.8 Ma. Therefore, the syenite complex and lamprophyre intruded into this suture zone during the same magmatic stage. The syenite complex and the lamprophyre have significant character with high-K (K2O/Na2O>3) and high-(LREE/HREE) ratios (normalized La/Yb>10) with SiO2 contents ranging from 40 to 65 wt%. In addition, the syenite and lamprophyre possess the relatively enriched epsilon Nd values (+1 to -1.5) rather than the pre-existent metamorphosed intrusive rocks (1000 to 770 Ma; epsilon Nd values from +6 to +1) in the southwest terrane.
Considering petrography and geochemistry, the syenite complex was originated from the lamprophyre magma, and fractional crystallization and accumulation played an important role of formation of the layered structure. The primitive composition of the lamprophyre contains phenocrysts of Mg- and Cr-rich phlogopite in equilibrium with K-rich basaltic magma. The syenite magma emplaced at the upper crustal levels (<300 MPa) during the ascent of the crust. These features suggest that the high-K mafic magma was produced by partial melting of the lithospheric mantle that was altered by subduction and/or collisional processes. The upwelling of asthenosphere under the extensional setting would be acted as the heat source to cause the partial melting of lithosphere. As the petrogenesis of the Cambrian Yamato syenite, 300 km west of the SRM, resembles that of the syenite complex from the SRM, similar petrogenetic model may also apply to the origin of high-K mafic magma in the Gondwana suture zone during the Pan-African stage.