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Petrogenesis of sector-zoned garnet in graphitic metapelite from the Danba dome, eastern Tibetan Plateau (SW China)

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Abstract

Texturally and chemically sector-zoned garnet crystals in two contiguous metapelitic rocks from the Danba dome, eastern Tibetan Plateau (SW China) were investigated. A petrographic boundary in one of the rocks (sample 21DB103) separates a thin section into two zones. Whereas one zone containing sector-zoned garnet and fined-grained matrix is enriched in graphite and quartz, the other zone encompasses garnets with relatively regular habit in a coarse-grained matrix poor in graphite and quartz. The two zones are distinct with regards to the chemical compositions of biotite and plagioclase, as well as the major and trace element zoning patterns of garnet. Electron back-scattered diffraction analysis shows that all the investigated garnet crystals in this sample are single crystals. Relatively higher P-T conditions are estimated for the initial growth of sector-zoned garnet (~ 5.0 kbar / ~540 ℃) compared to the regular garnet (~ 3.8 kbar / ~510 ℃) in this rock, possibly indicating that growth of the sector-zoned garnet postdates growth of the regular garnet. Texturally and chemically radial sectors with garnet-quartz intergrowths and irregular sectors of garnet are preserved in the other graphite-rich rock (sample 21DB104). Isopleth thermobarometry applied to the core of the largest garnet crystal exhibiting sector zoning in this sample reveals P-T conditions of initial garnet crystallization (~ 4.4 kbar / ~512 ℃) that deviate far (~ 0.8 kbar/~45 ℃) from equilibrium, potentially indicating significant overstepping required for garnet nucleation. Plagioclase inclusions in garnet display varying trace element abundances, indicating their replacements of different preexisting phases. These results suggest that abundant graphite may play a pivotal role in changing fluid conditions and reducing the solubility of SiO2 to grow sector-zoned garnet, as well as impeding matrix coarsening. Development of sector-zoned core and dodecahedral faces of garnet may be related to rapid growth with changes in crystal morphology. Irregular sectors may have developed through fluid infiltration and local chemical adjustments.

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Acknowledgements

We sincerely appreciate Yong-Hong Shi, Juan Wang and Glenn Poirier for their help with EPMA analyses. Bo Zhang and Hai-Ping Ren are thanked for their assistance with TIMA analysis. Fenghua Liang and Xiaomin Wang are thanked for their help with EBSD analysis. Fangyue Wang is acknowledged for his help with processing trace element data using LIMS. Constructive comments by three anonymous reviewers and the editorial work by Dante Canil are gratefully acknowledged. This work was supported by the National Natural Science Foundation of China (42330303, 42172054) and the China Scholarship Council (202104910335).

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Correspondence to C.-M. Wu.

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Communicated by Dante Canil.

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Li, Z.M., Gaidies, F., Chen, YC. et al. Petrogenesis of sector-zoned garnet in graphitic metapelite from the Danba dome, eastern Tibetan Plateau (SW China). Contrib Mineral Petrol 179, 56 (2024). https://doi.org/10.1007/s00410-024-02139-8

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  • DOI: https://doi.org/10.1007/s00410-024-02139-8

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