11:15 AM - 11:30 AM
[SMP22-09] Timing of garnet-forming metamorphism constrained by U–Th–Pb electron microprobe dating of monazite in a pelitic gneiss from Mefjell, Sør Rondane Mountains, East Antarctica
Keywords:monazite, garnet, petrochronology, Sør Rondane Mountains
The Sør Rondane Mountains (SRM) of eastern Dronning Maud Land, East Antarctica are considered to be located at the crossing point of the East African Orogen and Kuunga Orogen at ca. 750-530 Ma [1-3], and thus are the key area to understand the formation process of Gondwana. The SRM are divided into the NE terrane mainly composed of granulites with clockwise P–T–t path, and the SW terrane composed of granulites and lower-grade metamorphic rocks with counter-clockwise P–T–t path [4]. Based on these data, [4] proposed the tectonic model that the NE terrane thrusted up to the SW terrane. However, recent studies have reported P–T–t path inconsistent with this tectonic model [e.g., 5]. Therefore, it is important to obtain detailed pressure-temperature-deformation-time (P–T–D–t) paths of the metamorphic rocks more widely from the entire SRM in order to understand the collision process of the Gondwana supercontinent.
Mefjell from the central SRM has been considered to belong to the SW terrane [4], although hair-pin-shaped clockwise P–T–t path was reported [5]. Nakano et al. [6] also proposed a clockwise P–T path from a sillimanite-biotite-garnet gneiss (sampleTK2019122301A) from Mefjell by using the Zr-in-rutile geothermometer [7] that is robust to elemental diffusion at high-temperatures and retrograde re-equilibrium. Tsubokawa et al. [5] reported 700-540 Ma as the timing of peak metamorphism by electron microprobe (EMP) U–Th–Pb dating of matrix monazite.
In this study, we aim to constrain the timing of peak metamorphism and retrograde metamorphism in Mefjell through EMP U–Th–Pb dating of monazite in TK2019122301A using petrochronological approach. The porphyroblastic garnet in the studied sample has fractures filled with secondary biotite, quartz, plagioclase and muscovite. Some plagioclase grains enclosed in garnet are arranged parallel to the fractures, and have chemical zoning that may have formed through fluid infiltration at the timing of retrograde metamorphism. The porphyroblastic garnet has discontinuous chemical zoning in P that diffuses slowly in garnet. Using P zonings, garnet is divided into P-poor inner core, P-rich outer core, P-poor mantle, and moderately P-bearing rim [6].
Monazite occurs as inclusions in the garnet inner core, outer core, and rim, and is also present in the matrix [8]. Monazite occurring in the matrix has bright core and dark rim in BSE images [8]. The BSE-bright core shows low Y and M-HREE concentration (mostly Y2O3 <0.5 wt%) and has two age peaks of ca. 550 Ma and ca. 650 Ma. The BSE-dark rim shows high Y and M-HREE concentration (mostly Y2O3 >1.5 wt%) and has single age peak of ca. 550 Ma. The high Y concentration in monazite probably reflects disequilibrium with garnet. Therefore, ca. 550 Ma is likely the timing of retrograde metamorphism accompanied by garnet breakdown. On the other hand, the low-Y domain was probably in equilibrium with garnet which prefers Y [e.g., 9]. Therefore, the matrix monazite core probably grew at ca. 650 Ma in equilibrium with garnet, and partly rejuvenated during the retrograde metamorphism.
Monazite included in the garnet inner core and outer core does not show core and rim structure under BSE images and shows two age peaks of ca. 550 Ma and ca. 650 Ma. Monazite included in the garnet rim has several zones in BSE image but shows single age peak of ca. 650 Ma. Monazite included in the garnet outer core and rim has low Y concentration (mostly Y2O3 <1.5 wt%), whereas monazite included in the garnet inner core varies in Y concentration (Y2O3 0.1-3.9 wt%). Therefore, similar to the monazite in the matrix, we consider that the monazite inclusion in garnet was also influenced by fluid infiltration during the retrograde metamorphism at ca. 550 Ma and partly rejuvenated.
In summary, we consider that garnet grew at ca. 650 Ma, and garnet breakdown occurred during the retrograde metamorphism at ca. 550 Ma.
References
[1] Jacobs et al. 2003 Precam. Res. [2] Meert 2003 Tectonophysics [3] Satish-Kumar et al. 2013 Precam. Res. [4] Osanai et al. 2013 Precam. Res. [5] Tsubokawa et al. 2017 JMPS [6] Nakano et al. 2023 JpGU abst. [7] Tomkins et al. 2007 JMG [8] Nakano et al. 2023 GSJ abst. [9] Foster et al. 2000 EPSL
Mefjell from the central SRM has been considered to belong to the SW terrane [4], although hair-pin-shaped clockwise P–T–t path was reported [5]. Nakano et al. [6] also proposed a clockwise P–T path from a sillimanite-biotite-garnet gneiss (sampleTK2019122301A) from Mefjell by using the Zr-in-rutile geothermometer [7] that is robust to elemental diffusion at high-temperatures and retrograde re-equilibrium. Tsubokawa et al. [5] reported 700-540 Ma as the timing of peak metamorphism by electron microprobe (EMP) U–Th–Pb dating of matrix monazite.
In this study, we aim to constrain the timing of peak metamorphism and retrograde metamorphism in Mefjell through EMP U–Th–Pb dating of monazite in TK2019122301A using petrochronological approach. The porphyroblastic garnet in the studied sample has fractures filled with secondary biotite, quartz, plagioclase and muscovite. Some plagioclase grains enclosed in garnet are arranged parallel to the fractures, and have chemical zoning that may have formed through fluid infiltration at the timing of retrograde metamorphism. The porphyroblastic garnet has discontinuous chemical zoning in P that diffuses slowly in garnet. Using P zonings, garnet is divided into P-poor inner core, P-rich outer core, P-poor mantle, and moderately P-bearing rim [6].
Monazite occurs as inclusions in the garnet inner core, outer core, and rim, and is also present in the matrix [8]. Monazite occurring in the matrix has bright core and dark rim in BSE images [8]. The BSE-bright core shows low Y and M-HREE concentration (mostly Y2O3 <0.5 wt%) and has two age peaks of ca. 550 Ma and ca. 650 Ma. The BSE-dark rim shows high Y and M-HREE concentration (mostly Y2O3 >1.5 wt%) and has single age peak of ca. 550 Ma. The high Y concentration in monazite probably reflects disequilibrium with garnet. Therefore, ca. 550 Ma is likely the timing of retrograde metamorphism accompanied by garnet breakdown. On the other hand, the low-Y domain was probably in equilibrium with garnet which prefers Y [e.g., 9]. Therefore, the matrix monazite core probably grew at ca. 650 Ma in equilibrium with garnet, and partly rejuvenated during the retrograde metamorphism.
Monazite included in the garnet inner core and outer core does not show core and rim structure under BSE images and shows two age peaks of ca. 550 Ma and ca. 650 Ma. Monazite included in the garnet rim has several zones in BSE image but shows single age peak of ca. 650 Ma. Monazite included in the garnet outer core and rim has low Y concentration (mostly Y2O3 <1.5 wt%), whereas monazite included in the garnet inner core varies in Y concentration (Y2O3 0.1-3.9 wt%). Therefore, similar to the monazite in the matrix, we consider that the monazite inclusion in garnet was also influenced by fluid infiltration during the retrograde metamorphism at ca. 550 Ma and partly rejuvenated.
In summary, we consider that garnet grew at ca. 650 Ma, and garnet breakdown occurred during the retrograde metamorphism at ca. 550 Ma.
References
[1] Jacobs et al. 2003 Precam. Res. [2] Meert 2003 Tectonophysics [3] Satish-Kumar et al. 2013 Precam. Res. [4] Osanai et al. 2013 Precam. Res. [5] Tsubokawa et al. 2017 JMPS [6] Nakano et al. 2023 JpGU abst. [7] Tomkins et al. 2007 JMG [8] Nakano et al. 2023 GSJ abst. [9] Foster et al. 2000 EPSL