1:45 PM - 2:00 PM
[SMP28-13] Combination of EBSD analysis and zircon U–Pb geochronology: Recognition of “psuedo-inclusions” and multi-stage deformation in zircon of Sanbagawa eclogite
Keywords:zircon, U–Pb, Sanbagawa belt, inclusion, plastic deformation
Zircon U-Pb dating is one of the most useful methods for age determination of metamorphic rocks due to the associated high closure temperature and the physical stability of zircon. By combining age data with mineral inclusions in zircon, it is possible to directly relate ages with metamorphic history. However, the presence of subgrain boundaries or cracks caused by deformation may lead to Pb loss and facilitate the development of “pseudo-inclusions”. Hence, the interpretation of zircon U-Pb ages requires careful consideration. Zircon geochronology plays an important role in analyzing orogeny. However, to reveal the associated metamorphic history, it is necessary to discuss micro-textures of zircon in combination with chronological data. Pb loss of younger zircon is commonly difficult to identify, and the identification of “pseudo-inclusions” (inclusions that formed after the host crystal that have been introduced along cracks etc.) is generally based on visual inspection of cathodoluminescence (CL) images and its reliability is not always assured. This study evaluates the potential impact of deformation on the formation of zircon inclusions by combining U-Pb dating by laser ablation (LA)-ICP-MS with intragrain strain distribution analysis using electron backscatter diffraction (EBSD).
The analyzed zircon grains were separated from eclogite of the Seba metagabbro body, part of the Eclogite Unit in the Besshi area, Shikoku. In CL images the zircon grains are composed of zoning or cloudy core domains and homogeneous rim domains. Concordant 238U ages obtained from the core domains range from 179 to 199 Ma. The REE patterns of core domains show negative Eu anomalies and enrichment in heavy REE, indicating growth of zircon in the absence of garnet in the host rock. Th/U ratios show values characteristic for igneous zircon.
These zircon grains were also studied using EBSD and Grain Reference Orientation Deviation (GROD) and Kernel Average Misorientation (KAM) maps constructed. A comparison of these EBSD maps with images obtained from optical microscopy, CL, and backscattered electrons (BS), three types of deformation structures were identified: (1) cracks formed by brittle deformation, (2) subgrain boundaries associated with plastic deformation, and (3) trails of voids and fluid inclusions. Cracks were most clearly observed in optical and BS images but are commonly indistinct in CL images and KAM maps. Subgrain boundaries were recognized in KAM maps but were not seen in optical and BS images. Trails of voids are visible in optical and CL images but not in KAM maps. A combination of zircon microstructural observations suggests the following sequence of events: growth of zircon cores → formation of trails of voids → overgrowth of metamorphic rim → development of cracks and subgrain boundaries. The distribution of inclusions in zircon is closely associated with the deformation structures. Notably, omphacite, an indicator of eclogite-facies metamorphism, was found only along cracks or subgrain boundaries. This suggests that these inclusions in zircon are not primary inclusions captured during zircon growth but were introduced secondarily along micro-cracks formed during post-growth deformation. Despite the presence of mineral inclusions indicative of high-pressure metamorphism in some zircon grains, the zircon U-Pb ages of 180-200 Ma is interpreted as the igneous protolith age of the Seba metagabbro, and not a metamorphic age. This interpretation is supported by the trace element data.
This study demonstrates that combining intragrain strain distribution analysis using EBSD with CL observations can help distinguish "pseudo-inclusions" introduced during post-growth deformation in zircon and identify multiple deformation stages within zircon. The application of EBSD to zircon geochronology is a powerful tool for linking deformation structures to metamorphic and deformation histories, providing a more robust interpretation of zircon ages.
The analyzed zircon grains were separated from eclogite of the Seba metagabbro body, part of the Eclogite Unit in the Besshi area, Shikoku. In CL images the zircon grains are composed of zoning or cloudy core domains and homogeneous rim domains. Concordant 238U ages obtained from the core domains range from 179 to 199 Ma. The REE patterns of core domains show negative Eu anomalies and enrichment in heavy REE, indicating growth of zircon in the absence of garnet in the host rock. Th/U ratios show values characteristic for igneous zircon.
These zircon grains were also studied using EBSD and Grain Reference Orientation Deviation (GROD) and Kernel Average Misorientation (KAM) maps constructed. A comparison of these EBSD maps with images obtained from optical microscopy, CL, and backscattered electrons (BS), three types of deformation structures were identified: (1) cracks formed by brittle deformation, (2) subgrain boundaries associated with plastic deformation, and (3) trails of voids and fluid inclusions. Cracks were most clearly observed in optical and BS images but are commonly indistinct in CL images and KAM maps. Subgrain boundaries were recognized in KAM maps but were not seen in optical and BS images. Trails of voids are visible in optical and CL images but not in KAM maps. A combination of zircon microstructural observations suggests the following sequence of events: growth of zircon cores → formation of trails of voids → overgrowth of metamorphic rim → development of cracks and subgrain boundaries. The distribution of inclusions in zircon is closely associated with the deformation structures. Notably, omphacite, an indicator of eclogite-facies metamorphism, was found only along cracks or subgrain boundaries. This suggests that these inclusions in zircon are not primary inclusions captured during zircon growth but were introduced secondarily along micro-cracks formed during post-growth deformation. Despite the presence of mineral inclusions indicative of high-pressure metamorphism in some zircon grains, the zircon U-Pb ages of 180-200 Ma is interpreted as the igneous protolith age of the Seba metagabbro, and not a metamorphic age. This interpretation is supported by the trace element data.
This study demonstrates that combining intragrain strain distribution analysis using EBSD with CL observations can help distinguish "pseudo-inclusions" introduced during post-growth deformation in zircon and identify multiple deformation stages within zircon. The application of EBSD to zircon geochronology is a powerful tool for linking deformation structures to metamorphic and deformation histories, providing a more robust interpretation of zircon ages.