5:15 PM - 7:15 PM
[SCG54-P13] Structural and geochemical analysis of a metasomatized tectonic mélange in the Nomo metamorphic rocks, western Kyusyu, Japan.
Keywords:metasomatism, Nagasaki metamorphic rocks, tectonic mélange, dissolution-precipitation creep
The slab-mantle interface zone is known to be a site where rocks derived from the subducting oceanic crust and the mantle wedge are mechanically mixed (Bebout and Penniston-Dorland, 2016). In such plate boundary zones, the slab- and mantle wedge-derived materials occur as tectonic blocks, while the matrix surrounding these blocks is composed of hydrous minerals that formed via metasomatic reactions in the presence of slab-derived fluids (Tarling et al, 2019). In addition, since such metasomatic reactions involve dehyration, hydrofracturing may have occurred due to elevated pore fluid pressures (Nishiyama et al., 2017). In the Nomo metamorphic rocks, western Kyushu, Japan, a tectonic mélange formed through CaO- and Na2O-metasomatism occurs as a shear zone ranging from a few meters to several tens of meters in thickness at the boundary between serpentinite bodies and basic schists (Nishiyama et al., 1997). In this study, we conducted structural and geochemical analyses to understand the formation processes of the tectonic mélange in the Nomo metamorphic rocks.
The tectonic mélange exhibits a block-in-matrix structure, characterized by lenticular blocks consisting mainly of basic schists that are enclosed within a metasomatized matrix. Some blocks are hexagonal in shape, suggesting that they resulted from the development of a network of conjugate extensional and extensional-shear fractures at near-lithostatic pore fluid pressures (Sibson, 2017). The matrix is primarily composed of actinolite, chlorite, epidote, and albite. Actinolite- or chlorite-rich aggregates form a localized shear zone between the fractured blocks. The fine-grained actinolite aggregates exhibit a strong crystal-preferred orientation (CPO), with (100) poles oriented perpendicular to the schistosity, (010) poles perpendicular to the lineation within the schistosity plane, and [001] axes parallel to the lineation. Furthermore, misorientation angle distributions between neighboring grain pairs show a high frequency of misorientation angles <15°, suggesting the presence of low-angle or subgrain boundaries. These microstructural features suggest that dynamic recrystallization by subgrain rotation has occurred via dislocation creep. The actinolite aggregates exhibit compositional zoning with a decrease in Al content from core to rim. This suggest that in addition to dislocation creep, the actinolite aggregates have undergone dissolution-precipitation creep, in which grain rotation along with anisotropic grain growth contributes to the development of a strong CPO (Lee et al., 2022).
Whole-rock major and trace element analyses of matrix-supported tectonic mélange and basic schist samples, combined with mass balance calculations using the isocon method, reveal that the mélange has lower CaO and higher Cr, MgO, and Na2O contents compared to the basic schist. Furthermore, mineral chemistry data indicate that chlorite in the matrix has higher Cr and MgO concentrations than that in the blocks (basic schist). Since Cr and MgO are more abundant in mantle peridotites, these geochemical characteristics, together with field and microstructural observations, suggest that Cr- and Mg-rich fluids derived from serpentinization of mantle wedge peridotites migrated and then infiltrated the basic schist through a network of extensional and extensional-shear fractures (i.e., fault-fracture mesh). This fluid infiltration likely facilitated the precipitation of Cr- and MgO-rich chlorite aggregates through dissolution-precipitation creep.
References: Bebout and Penniston-Dorland (2016), Lithos, 240-243, 228-258. Lee et al. (2022), J. Struct. Geol., 155, 104505, Nishiyama et al. (1997), The 104th Annual Meeting Geological Society of Japan, Excursion Guidebook, 131-162. Nishiyama et al. (2017), Prog. Earth Planet. Sci., 4, 1-17. Sibson (2017), Earth Planets Space, 69, 113. Tarling et al. (2019), Nat. Geosci., 12, 1034-1042.
The tectonic mélange exhibits a block-in-matrix structure, characterized by lenticular blocks consisting mainly of basic schists that are enclosed within a metasomatized matrix. Some blocks are hexagonal in shape, suggesting that they resulted from the development of a network of conjugate extensional and extensional-shear fractures at near-lithostatic pore fluid pressures (Sibson, 2017). The matrix is primarily composed of actinolite, chlorite, epidote, and albite. Actinolite- or chlorite-rich aggregates form a localized shear zone between the fractured blocks. The fine-grained actinolite aggregates exhibit a strong crystal-preferred orientation (CPO), with (100) poles oriented perpendicular to the schistosity, (010) poles perpendicular to the lineation within the schistosity plane, and [001] axes parallel to the lineation. Furthermore, misorientation angle distributions between neighboring grain pairs show a high frequency of misorientation angles <15°, suggesting the presence of low-angle or subgrain boundaries. These microstructural features suggest that dynamic recrystallization by subgrain rotation has occurred via dislocation creep. The actinolite aggregates exhibit compositional zoning with a decrease in Al content from core to rim. This suggest that in addition to dislocation creep, the actinolite aggregates have undergone dissolution-precipitation creep, in which grain rotation along with anisotropic grain growth contributes to the development of a strong CPO (Lee et al., 2022).
Whole-rock major and trace element analyses of matrix-supported tectonic mélange and basic schist samples, combined with mass balance calculations using the isocon method, reveal that the mélange has lower CaO and higher Cr, MgO, and Na2O contents compared to the basic schist. Furthermore, mineral chemistry data indicate that chlorite in the matrix has higher Cr and MgO concentrations than that in the blocks (basic schist). Since Cr and MgO are more abundant in mantle peridotites, these geochemical characteristics, together with field and microstructural observations, suggest that Cr- and Mg-rich fluids derived from serpentinization of mantle wedge peridotites migrated and then infiltrated the basic schist through a network of extensional and extensional-shear fractures (i.e., fault-fracture mesh). This fluid infiltration likely facilitated the precipitation of Cr- and MgO-rich chlorite aggregates through dissolution-precipitation creep.
References: Bebout and Penniston-Dorland (2016), Lithos, 240-243, 228-258. Lee et al. (2022), J. Struct. Geol., 155, 104505, Nishiyama et al. (1997), The 104th Annual Meeting Geological Society of Japan, Excursion Guidebook, 131-162. Nishiyama et al. (2017), Prog. Earth Planet. Sci., 4, 1-17. Sibson (2017), Earth Planets Space, 69, 113. Tarling et al. (2019), Nat. Geosci., 12, 1034-1042.