Growth of plagioclase rims around metastable kyanite during decompression of high-pressure felsic granulites (Bohemian Massif)

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Abstract

Samples of high-pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) characterized by a peak metamorphic (high-pressure) mineral assemblage of garnet–kyanite–plagioclase–K-feldspar–quartz ± biotite show well-developed plagioclase reaction rims around kyanite grains in two microstructural settings. In one setting, kyanite is randomly distributed in the polyphase matrix, whereas in the other setting, it is enclosed within large perthitic K-feldspar. Kyanite is regarded as a relict of the high-pressure metamorphic assemblage that became metastable during transition to a low-pressure overprint. Plagioclase rims from both microstructural settings show continuous outwards decrease of the anorthite content from An32–25 at the contact with kyanite to An20–19 at the contact with the matrix or to the perthitic K-feldspar respectively. Based on mass balance considerations, it is shown that in some cases, a small amount of kyanite was consumed in the rim-forming reaction to provide the Al2O3 component for the growth of plagioclase, whereas in other cases no Al2O3 from kyanite was necessary. In a majority of examples, the necessary Al2O3 was supplied with CaO and Na2O from the surrounding matrix material. For kyanite in perthite, a thermodynamic analysis reveals that the kyanite became metastable at the interface with the host perthite at the peak metamorphic pressure, and therefore the plagioclase rim started to grow at ˜ 18 kbar. In contrast, kyanite in the polyphase matrix remained stable down to pressures of ˜ 16 kbar, and the plagioclase rim only started to grow at a later stage during the decompression. Plagioclase rims around kyanite inclusions within large perthite have a radial thickness of up to 50 μm. In contrast, the radial thickness of plagioclase rims around kyanite in the polycrystalline matrix is significantly larger, up to 200 μm. Another peculiarity is that the plagioclase rims around kyanite in the matrix are polycrystalline, whereas the plagioclase rims around kyanite inclusions in perthitic hosts are single crystals with the same crystallographic orientation as the host perthite. The difference in rim thickness for the two microstructural settings is ascribed to the differences in the efficiency of chemical mass transfer next to the reaction site. The comparatively large thickness of the plagioclase rims grown around kyanite in the matrix is probably due to efficient material transport along the grain and phase boundaries in the matrix. In contrast, chemical mass transfer was comparatively slow in the large perthitic K-feldspar grains.

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