2:30 PM - 2:45 PM
[MIS13-15] Recent research on hard rock geology in the regions of Japanese Antarctic Research Expedition
Keywords:Antarctica, Continental collision zone, Continental crust, Partial melting, Geofluids, Magma
Recent hard rock geology by JARE have mainly focused on metamorphic and igneous rocks exposed in the Lützow-Holm Complex (LHC) and the Sør Rondane Mountains (SRM), East Antarctica. This area is located at the intersection of the East African Antarctic Orogenic Belt and the Kuunga Orogenic Belt, where the unresolved question of the overprinting of two orogenies during the formation of the Gondwana can be tested [1]. Our research is twofold: (1) study on formation tectonics of the study area and (2) studies on partial melting, magma generation processes and geofluid activities in the middle to lower crust. By understanding of formation tectonics can we understand the location where the processes studied in (2) are ongoing in today’s Earth.
The LHC is a high-grade metamorphosed area of 630-520 Ma ranging from upper-amphibolite to granulite facies, with the highest-grade reaching ultra-high temperature (UHT) metamorphism. Recently, detrital zircon datings found suites of different protolith ages in the LHC, and 1 Ga block free from ca. 600-500 Ma metamorphic overprint was found along the Prince Olav Coast [4]. In addition, although the metamorphic zone mapping based on the matrix mineral assemblages show progressive increase of metamorphic grade in the granulite facies region, recent analysis based on garnet inclusions reveals that the granulite facies region, excluding the UHT point, experienced approximately the same T conditions at the P equivalent to the lower crust [5].
Although the formation tectonics of the LHC is yet to be understood, progress has been made in understanding the formation mechanism of the UHT rocks. [6] showed from P-T-t path analysis using inclusions in zircon and garnet that the prograde path of the UHT rock is a clockwise one with P increase and high-T duration is 40 Myr. Furthermore, the composition of nanogranite included in zircon was determined after remelting by piston cylinder experiments in addition to the glass inclusions in garnet. [6] found that the compositional change of the melt can be explained by the equilibrium melting in closed system accompanied with P and T increase. From the above results, [6] considered UHT metamorphism in the LHC had a slow heating process and radioactive heat was the main heat source.
The SRM exposes a 650-520 Ma collision boundary as the MTB. The hanging-wall of the MTB is termed the NE terrane, where granulites with clockwise P-T paths and UHT metamorphic rocks are reported [7, 8]. The footwall is termed the SW terrane, where granulites and lower-T metamorphic rocks with counterclockwise P-T paths are exposed [7]. [7] proposed a tectonic model in which the NE terrane thrusts up onto the SW terrane at 650-600 Ma. On the other hand, [9] proposed a model in which the Namuno terrane in Africa thrusted up onto the Nampula terrane along a mega-thrust that was active at 580-540 Ma and considered that the SRM belongs to the hanging-wall side. Recently, clockwise P-T-t paths have been reported from several outcrops of the SW terrane, and the polymetamorphism as well as the tectonic models are being tested. [10] proposed the existence of a low-angle tectonic boundary in the central SRM apart from the MTB, reporting that its hanging-wall experienced ~600 Ma metamorphism while its footwall experienced 550 Ma metamorphism alone.
Saline fluid activities in the middle to lower crust have been studied extensively in the SRM. [11] reported the localized distribution of high-Cl minerals along the MTB and discussed mass transport using diffusion patterns left in the wall rocks formed by the infiltration of saline fluids along cracks and shear zones during the regrade metamorphism. The origin of the fluid is seawater or fluids released from granite or pegmatite during solidification [12]. [13] applied a reactive transport model based on advection-diffusion equations to similar structures and estimated the duration of fluid influx shorter than 12 hours. [14] utilized a drone to analyze the orientations of pegmatite veins around a granite pluton to reconstruct the stress field change. On the other hand, saline fluids infiltrated during the prograde metamorphism had low O isotope composition possibly derived from mafic rocks [15].
As above, the LHC and SRM are excellent fields for studying deep crustal processes. We look forward to welcoming young scientists to Antarctic geological research.
[1]Satish-Kumar+ 2013 PR [4]Dunkley+ 2020 PS [5]Suzuki & Kawakami 2019 JMPS [6] Suzuki+ 2023 JpGU [7]Osanai+ 2013 PR [8]Higashino & Kawakami 2022 JMPS [9] Grantham+ 2013 PR [10]Adachi+ 2023 JMPS in review [11]Higashino+ 2019 JPet [12]Kawakami+ 2022 NIPR abst [13]Mindaleva+ 2020 Lithos [14]Uno+ 2022 JpGU [15]Higashino+ 2019 JMG
The LHC is a high-grade metamorphosed area of 630-520 Ma ranging from upper-amphibolite to granulite facies, with the highest-grade reaching ultra-high temperature (UHT) metamorphism. Recently, detrital zircon datings found suites of different protolith ages in the LHC, and 1 Ga block free from ca. 600-500 Ma metamorphic overprint was found along the Prince Olav Coast [4]. In addition, although the metamorphic zone mapping based on the matrix mineral assemblages show progressive increase of metamorphic grade in the granulite facies region, recent analysis based on garnet inclusions reveals that the granulite facies region, excluding the UHT point, experienced approximately the same T conditions at the P equivalent to the lower crust [5].
Although the formation tectonics of the LHC is yet to be understood, progress has been made in understanding the formation mechanism of the UHT rocks. [6] showed from P-T-t path analysis using inclusions in zircon and garnet that the prograde path of the UHT rock is a clockwise one with P increase and high-T duration is 40 Myr. Furthermore, the composition of nanogranite included in zircon was determined after remelting by piston cylinder experiments in addition to the glass inclusions in garnet. [6] found that the compositional change of the melt can be explained by the equilibrium melting in closed system accompanied with P and T increase. From the above results, [6] considered UHT metamorphism in the LHC had a slow heating process and radioactive heat was the main heat source.
The SRM exposes a 650-520 Ma collision boundary as the MTB. The hanging-wall of the MTB is termed the NE terrane, where granulites with clockwise P-T paths and UHT metamorphic rocks are reported [7, 8]. The footwall is termed the SW terrane, where granulites and lower-T metamorphic rocks with counterclockwise P-T paths are exposed [7]. [7] proposed a tectonic model in which the NE terrane thrusts up onto the SW terrane at 650-600 Ma. On the other hand, [9] proposed a model in which the Namuno terrane in Africa thrusted up onto the Nampula terrane along a mega-thrust that was active at 580-540 Ma and considered that the SRM belongs to the hanging-wall side. Recently, clockwise P-T-t paths have been reported from several outcrops of the SW terrane, and the polymetamorphism as well as the tectonic models are being tested. [10] proposed the existence of a low-angle tectonic boundary in the central SRM apart from the MTB, reporting that its hanging-wall experienced ~600 Ma metamorphism while its footwall experienced 550 Ma metamorphism alone.
Saline fluid activities in the middle to lower crust have been studied extensively in the SRM. [11] reported the localized distribution of high-Cl minerals along the MTB and discussed mass transport using diffusion patterns left in the wall rocks formed by the infiltration of saline fluids along cracks and shear zones during the regrade metamorphism. The origin of the fluid is seawater or fluids released from granite or pegmatite during solidification [12]. [13] applied a reactive transport model based on advection-diffusion equations to similar structures and estimated the duration of fluid influx shorter than 12 hours. [14] utilized a drone to analyze the orientations of pegmatite veins around a granite pluton to reconstruct the stress field change. On the other hand, saline fluids infiltrated during the prograde metamorphism had low O isotope composition possibly derived from mafic rocks [15].
As above, the LHC and SRM are excellent fields for studying deep crustal processes. We look forward to welcoming young scientists to Antarctic geological research.
[1]Satish-Kumar+ 2013 PR [4]Dunkley+ 2020 PS [5]Suzuki & Kawakami 2019 JMPS [6] Suzuki+ 2023 JpGU [7]Osanai+ 2013 PR [8]Higashino & Kawakami 2022 JMPS [9] Grantham+ 2013 PR [10]Adachi+ 2023 JMPS in review [11]Higashino+ 2019 JPet [12]Kawakami+ 2022 NIPR abst [13]Mindaleva+ 2020 Lithos [14]Uno+ 2022 JpGU [15]Higashino+ 2019 JMG