Presentation Information
[R8-02]Possible ultra-high temperature metamorphism recorded by antiperthitic ternary feldspar in the Greater Himalayan Crystalline Complex, Marsyangdi River, Central Nepal
*Siqi Zhao1, Tetsuo Kawakami1, Fumiko Higashino1 (1. Kyoto University. Sci.)
Keywords:
Greater Himalayan Crystalline Complex,ultra-high temperature metamorphism,ternary feldspar thermometry,antiperthitic ternary feldspar,phosphorus zoning
Recent reports of ultra-high temperature (UHT) metamorphism from the central to eastern Himalaya (e.g., Wang et al., 2021; Wu et al., 2022; Zhang et al., 2024) have challenged the traditional view that the Greater Himalayan Crystalline complex (GHC) mainly records amphibolite- to granulite-facies metamorphism. In this study, we report new evidence for UHT metamorphism from antiperthitic ternary feldspar in kyanite–sillimanite bearing gneiss collected from the GHC along the Marsyangdi River, central Nepal Himalaya. The studied samples, HMC06 and HMC07, are kyanite–sillimanite–garnet two-mica gneisses containing quartz, plagioclase, K-feldspar, garnet, biotite, muscovite, sillimanite, and kyanite. These samples were collected from the GHC Formation I on the west bank of the Marsyangdi River, north of the sillimanite-in isograd determined by Catlos et al. (2001).
Antiperthitic feldspar in these samples occurs as plagioclase host containing thick K-feldspar lamellae. The original ternary feldspar compositions were reconstructed by reintegrating the compositions and areal proportions of the plagioclase host and K-feldspar lamellae. The reintegrated compositions were plotted on ternary feldspar diagrams and compared with ternary feldspar solvus models and temperature fields after Fuhrman and Lindsley (1988), Kroll et al. (1993), and Benisek et al. (2004), at pressures of 0.8 and 1.0 GPa. For whole grain reintegration, HMC06 plots entirely above the 900 °C field, whereas most reintegrated compositions from HMC07 also indicate temperatures exceeding 900 °C.
In addition to major element analysis, EPMA trace element mapping revealed clear phosphorus (P) zoning in the antiperthitic feldspar. The K-feldspar lamellae bearing cores are relatively P-poor, whereas the surrounding rims are P-rich and generally lack K-feldspar lamellae. K-feldspar lamellae or inclusions locally present in the P-rich rims show higher relative P intensity than those in the P-poor cores. Based on this zoning pattern, the P-poor core was interpreted as the most appropriate domain for reconstructing the high-temperature ternary feldspar composition. Areal proportions of the plagioclase host and K-feldspar lamellae were estimated from backscattered electron (BSE) images using ImageJ color-threshold analysis. When only the P-poor cores were used for reintegration, all calculated compositions plotted above the 900 °C solvus, and the inferred temperatures were higher than those obtained from whole grain reintegration.
These results have methodological implications for ternary feldspar thermometry. Previous applications commonly estimated areal proportions from whole mesoperthitic or antiperthitic grains using optical or BSE images, without explicitly considering internal chemical zoning. Our results indicate that P zoning can provide an objective criterion for defining the effective reintegration domain, thereby reducing the risk of mixing chemically distinct domains that may not represent the same metamorphic stage. Therefore, P trace element mapping may offer a useful refinement for applying ternary feldspar thermometry to exsolved feldspar in high-grade metamorphic rocks.
Although additional antiperthite-bearing samples from the Marsyangdi River transect are required to exclude the inherited protolith origin of the UHT antiperthites (e.g., Koley et al., 2022), the above-mentioned results from HMC06 and HMC07 may indicate that UHT metamorphism occurred within this part of the GHC. These findings contribute to ongoing discussions on the spatial distribution and tectonic significance of Himalayan UHT metamorphism, and suggest that antiperthitic ternary feldspar may preserve peak or near-peak thermal conditions that are not fully recorded by conventional thermobarometric assemblages.
Antiperthitic feldspar in these samples occurs as plagioclase host containing thick K-feldspar lamellae. The original ternary feldspar compositions were reconstructed by reintegrating the compositions and areal proportions of the plagioclase host and K-feldspar lamellae. The reintegrated compositions were plotted on ternary feldspar diagrams and compared with ternary feldspar solvus models and temperature fields after Fuhrman and Lindsley (1988), Kroll et al. (1993), and Benisek et al. (2004), at pressures of 0.8 and 1.0 GPa. For whole grain reintegration, HMC06 plots entirely above the 900 °C field, whereas most reintegrated compositions from HMC07 also indicate temperatures exceeding 900 °C.
In addition to major element analysis, EPMA trace element mapping revealed clear phosphorus (P) zoning in the antiperthitic feldspar. The K-feldspar lamellae bearing cores are relatively P-poor, whereas the surrounding rims are P-rich and generally lack K-feldspar lamellae. K-feldspar lamellae or inclusions locally present in the P-rich rims show higher relative P intensity than those in the P-poor cores. Based on this zoning pattern, the P-poor core was interpreted as the most appropriate domain for reconstructing the high-temperature ternary feldspar composition. Areal proportions of the plagioclase host and K-feldspar lamellae were estimated from backscattered electron (BSE) images using ImageJ color-threshold analysis. When only the P-poor cores were used for reintegration, all calculated compositions plotted above the 900 °C solvus, and the inferred temperatures were higher than those obtained from whole grain reintegration.
These results have methodological implications for ternary feldspar thermometry. Previous applications commonly estimated areal proportions from whole mesoperthitic or antiperthitic grains using optical or BSE images, without explicitly considering internal chemical zoning. Our results indicate that P zoning can provide an objective criterion for defining the effective reintegration domain, thereby reducing the risk of mixing chemically distinct domains that may not represent the same metamorphic stage. Therefore, P trace element mapping may offer a useful refinement for applying ternary feldspar thermometry to exsolved feldspar in high-grade metamorphic rocks.
Although additional antiperthite-bearing samples from the Marsyangdi River transect are required to exclude the inherited protolith origin of the UHT antiperthites (e.g., Koley et al., 2022), the above-mentioned results from HMC06 and HMC07 may indicate that UHT metamorphism occurred within this part of the GHC. These findings contribute to ongoing discussions on the spatial distribution and tectonic significance of Himalayan UHT metamorphism, and suggest that antiperthitic ternary feldspar may preserve peak or near-peak thermal conditions that are not fully recorded by conventional thermobarometric assemblages.