Presentation Information
[R8P-07]Deformation processes and timing of melt-influenced shear zones at Niban Rock, Lützow–Holm Complex, East Antarctica
*Sreehari LAKSHMANAN1, Tatsuro Adachi2, Yuki Mori3, Tomokazu Hokada4 (1. Shimane University, 2. Kyushu university, 3. University of Hyogo, 4. NIPR)
Keywords:
Shear zone,Quartz,Monazite age,High temperature,East Antarctica
Niban Rock, located in the eastern part of the Lützow–Holm Complex, East Antarctica, preserves high-grade gneisses associated with reverse shear zones. Previous studies have reported Tonian metamorphism from Niban Rock and nearby areas, but the relationship between deformation, metamorphism, and mineral growth in the shear-zone rocks remains insufficiently constrained. In this study, we examine garnet–biotite gneiss and sillimanite–garnet–biotite gneiss collected from reverse shear zones at Niban Rock, using petrographic observation, microstructural analysis, EPMA mineral chemistry, and geothermometry. The studied samples show gneissic to migmatitic textures with local melanosome–leucosome separation. Quartz commonly exhibits high-temperature recrystallization microstructures, including grain-boundary-migration recrystallization, high-angle grain boundaries, and locally chessboard extinction. These microstructures indicate that the reverse shear zones were deformed under ductile, high-temperature conditions. Also, microstructures indicating the melt present deformation conditions are also present within the samples from the shear zone. Major mineral in the foliation is biotite and locally forms mica-fish structures.
Garnet shows different textural characteristics depending on its occurrence. Garnet in the melanosome is commonly elongate to asymmetric and contains abundant mineral inclusions, whereas garnet in the leucosome is generally more equant and inclusion-poor. Garnet compositions are almandine-rich and show only limited compositional variation between melanosome and leucosome. Garnet–biotite geothermometry gives temperatures of approximately 660–680 °C for melanosome and related gneisses, whereas leucosome domains yield lower temperatures of approximately 620–630 °C. Ti-in-biotite thermometry from matrix biotite gives temperatures of approximately 635–655 °C. These temperature estimates are broadly consistent with upper amphibolite-facies conditions, although they may partly reflect later re-equilibration during cooling.Monazite ages from sheared samples are dominated by ca. 930 Ma, with a minor younger population around ca. 550 Ma. Together with the microstructural and mineral chemical data, these results suggest that the main high-temperature deformation and metamorphic record of the reverse shear-zone rocks is Tonian in age, whereas the minor Cambrian ages may represent later overprinting. This study provides basic constraints on the deformation conditions and mineral growth history of reverse shear-zone rocks at Niban Rock.
Garnet shows different textural characteristics depending on its occurrence. Garnet in the melanosome is commonly elongate to asymmetric and contains abundant mineral inclusions, whereas garnet in the leucosome is generally more equant and inclusion-poor. Garnet compositions are almandine-rich and show only limited compositional variation between melanosome and leucosome. Garnet–biotite geothermometry gives temperatures of approximately 660–680 °C for melanosome and related gneisses, whereas leucosome domains yield lower temperatures of approximately 620–630 °C. Ti-in-biotite thermometry from matrix biotite gives temperatures of approximately 635–655 °C. These temperature estimates are broadly consistent with upper amphibolite-facies conditions, although they may partly reflect later re-equilibration during cooling.Monazite ages from sheared samples are dominated by ca. 930 Ma, with a minor younger population around ca. 550 Ma. Together with the microstructural and mineral chemical data, these results suggest that the main high-temperature deformation and metamorphic record of the reverse shear-zone rocks is Tonian in age, whereas the minor Cambrian ages may represent later overprinting. This study provides basic constraints on the deformation conditions and mineral growth history of reverse shear-zone rocks at Niban Rock.