16:00 〜 16:15
[SMP26-08] Clockwise pressure-temperature path from Mefjell, Sør Rondane Mountains East Antarctica
キーワード：セール・ロンダーネ山地、南極、組成ゾーニング、ザクロ石、温度圧力履歴、変成岩
The Sør Rondane Mountains (SRM) of eastern Dronning Maud Land, East Antarctica are considered to be located at the collision zone between East and West Gondwana, at ca 650-520 Ma [1]. It is located at the crossing point of the East Africa Antarctic Orogen and the Kuunga Orogen [2-4]. Therefore, construction of detailed pressure-temperature-deformation-time (P-T-D-t) path of the metamorphic rocks from the SRM is important to understand the collision process of the Gondwana supercontinent.
The SRM are divided into the NE terrane where granulites with clockwise P-T path are distributed and the SW terrane where granulites and lower-grade metamorphic rocks with counter-clockwise P-T path are distributed [1]. Mefjell in the central SRM is considered to belong to the SW terrane [1] while a clockwise P-T path has been recently reported [5]. However, Fe-Mg exchange geothermometer, which can easily be affected by the reequilibrium during retrograde metamorphism, was used in [5] to construct the clockwise P-T path. In this study, aiming to confirm the clockwise P-T path in Mefjell, we construct a P-T path by using Zr-in-rutile geothermometer [6] that is more robust to retrograde reequilibrium.
The studied sample is a sillimanite-biotite-garnet gneiss from Mefjell mainly composed of garnet, biotite, sillimanite, plagioclase, quartz and K-feldspar with secondary muscovite. Porphyroblastic garnet has discontinuous chemical zoning in P, which is a slowly diffusing element in garnet. The chemical zoning in P, therefore, can be used to indicate the isochronous surface of the garnet growth. Using P zonings, garnet is divided into P-poor inner core, P-rich outer core, P-poor mantle and moderately P-bearing rim. There are several P-poor patches whose P concentration sharply drops from the surrounding outer core.
Apatite and monazite are included in the garnet inner and outer cores and also present along fractures developed in garnet. They are not included in P-poor patches, mantle and rim. Xenotime is found in the garnet inner core and also present along the fractures in garnet, whereas it is rare in P-poor patches. This suggests that the P-poor patches were simultaneously formed with the garnet mantle. Sillimanite is included in garnet cores[F1] , mantle and rim. Plagioclase is included in garnet, present in the matrix, and filling fractures in garnet.
The P-T conditions of garnet inner core, mantle, rim formation stages were estimated by the Zr-in-rutile geothermometer [6] and the garnet-Al silicate-plagioclase (GASP) geobarometer [7] to rutile, sillimanite, plagioclase inclusions and surrounding garnet of each domain. The prograde metamorphic P-T conditions were estimated using the composition of garnet inner core and plagioclase inclusion (An30-43) included in the garnet inner core. Sillimanite that encloses Zn-bearing spinel (XMg = 0.31, ZnO = 16.9 wt%) is included in the inner core of garnet. This mineral structure is indicative of the Zn-bearing staurolite breakdown and can be approximated by
Fe-staurolite + quartz = almandine + sillimanite + H2O [8].
This reaction probably occurred during the inner core growth. By using the composition of plagioclase (An30) included near the boundary between the garnet inner and outer cores and the garnet composition next to it, the P-T conditions of the garnet inner core stage is estimated as 692 oC, 0.6 GPa. Subsequent decrease in pressure was presumed from garnet outer core by GASP geobarometer [7], because An content of plagioclase included in garnet outer core becomes higher from the center (An20-24) to the margin (An30-32). The temperature condition for the outer core was not constrained due to the absence of rutile inclusion, while rutile included in the garnet mantle gave almost the same temperature condition as the inner core stage. Therefore, we assumed that temperature condition of the garnet mantle stage was almost the same as that of the garnet outer core stage. Accordingly, the clockwise P-T path can be constructed.
We also reexamined electron microprobe U–Th–Pb monazite dating data reported in [5]. Focusing on chemical zoning in HREE, we obtained the weighted average age for monazite rim to be 572±9.8 Ma.
[1] Osanai et al. 2013 PR [2] Jacobs et al. 2003 PR [3] Meert 2003 Tectonophysics [4] Satish-Kumar et al. 2013 PR [5] Tsubokawa et al. 2017 JMPS [6] Tomkins et al. 2007 JMG [7] Holdaway 2001 AmMin [8] Spear & Cheney 1989 CMP
The SRM are divided into the NE terrane where granulites with clockwise P-T path are distributed and the SW terrane where granulites and lower-grade metamorphic rocks with counter-clockwise P-T path are distributed [1]. Mefjell in the central SRM is considered to belong to the SW terrane [1] while a clockwise P-T path has been recently reported [5]. However, Fe-Mg exchange geothermometer, which can easily be affected by the reequilibrium during retrograde metamorphism, was used in [5] to construct the clockwise P-T path. In this study, aiming to confirm the clockwise P-T path in Mefjell, we construct a P-T path by using Zr-in-rutile geothermometer [6] that is more robust to retrograde reequilibrium.
The studied sample is a sillimanite-biotite-garnet gneiss from Mefjell mainly composed of garnet, biotite, sillimanite, plagioclase, quartz and K-feldspar with secondary muscovite. Porphyroblastic garnet has discontinuous chemical zoning in P, which is a slowly diffusing element in garnet. The chemical zoning in P, therefore, can be used to indicate the isochronous surface of the garnet growth. Using P zonings, garnet is divided into P-poor inner core, P-rich outer core, P-poor mantle and moderately P-bearing rim. There are several P-poor patches whose P concentration sharply drops from the surrounding outer core.
Apatite and monazite are included in the garnet inner and outer cores and also present along fractures developed in garnet. They are not included in P-poor patches, mantle and rim. Xenotime is found in the garnet inner core and also present along the fractures in garnet, whereas it is rare in P-poor patches. This suggests that the P-poor patches were simultaneously formed with the garnet mantle. Sillimanite is included in garnet cores[F1] , mantle and rim. Plagioclase is included in garnet, present in the matrix, and filling fractures in garnet.
The P-T conditions of garnet inner core, mantle, rim formation stages were estimated by the Zr-in-rutile geothermometer [6] and the garnet-Al silicate-plagioclase (GASP) geobarometer [7] to rutile, sillimanite, plagioclase inclusions and surrounding garnet of each domain. The prograde metamorphic P-T conditions were estimated using the composition of garnet inner core and plagioclase inclusion (An30-43) included in the garnet inner core. Sillimanite that encloses Zn-bearing spinel (XMg = 0.31, ZnO = 16.9 wt%) is included in the inner core of garnet. This mineral structure is indicative of the Zn-bearing staurolite breakdown and can be approximated by
Fe-staurolite + quartz = almandine + sillimanite + H2O [8].
This reaction probably occurred during the inner core growth. By using the composition of plagioclase (An30) included near the boundary between the garnet inner and outer cores and the garnet composition next to it, the P-T conditions of the garnet inner core stage is estimated as 692 oC, 0.6 GPa. Subsequent decrease in pressure was presumed from garnet outer core by GASP geobarometer [7], because An content of plagioclase included in garnet outer core becomes higher from the center (An20-24) to the margin (An30-32). The temperature condition for the outer core was not constrained due to the absence of rutile inclusion, while rutile included in the garnet mantle gave almost the same temperature condition as the inner core stage. Therefore, we assumed that temperature condition of the garnet mantle stage was almost the same as that of the garnet outer core stage. Accordingly, the clockwise P-T path can be constructed.
We also reexamined electron microprobe U–Th–Pb monazite dating data reported in [5]. Focusing on chemical zoning in HREE, we obtained the weighted average age for monazite rim to be 572±9.8 Ma.
[1] Osanai et al. 2013 PR [2] Jacobs et al. 2003 PR [3] Meert 2003 Tectonophysics [4] Satish-Kumar et al. 2013 PR [5] Tsubokawa et al. 2017 JMPS [6] Tomkins et al. 2007 JMG [7] Holdaway 2001 AmMin [8] Spear & Cheney 1989 CMP