The metamorphic nappes of the Erzgebirge all include the same sequence of volcanosedimentary protoliths, but were metamorphosed to different extents during the Variscan orogeny. Metamorphic conditions range from very-low grade to eclogite facies. Geochemical and Sr-isotope data identify distinct protoliths, which can be linked to their unmetamorphosed equivalents from the former Gondwana shelf. Comparison of the Li and B contents and isotope ratios in metamorphic rocks from two distinctive metasedimentary units and their unmetamorphosed protoliths show that there is an overall loss of Li and B and and a decrease of δ7Li and δ11B during progressive metamorphism, but Li and B do not behave coherently. Additions from late fluids affect the B isotopic composition (shifting δ11B to higher values) more than the Li isotopic composition. The mobility of Li and B is dependent on the metamorphic history and pre-metamorphic composition of the rocks.In the sedimentary protoliths, exchangeable Li is mainly bound to illite and sericitic muscovite and structurally-bound Li is hosted in chlorite and illite/sericite, whereby chlorite-rich sedimentary rocks have distinctly higher Li contents. Structurally-bound Li has a narrow δ7Li range (−3.4 to +0.4), whereas samples with significant contributions of exchangeable Li have a broad range of δ7Li (−6 to +14.5) that depends on the depositional environment. In contrast, B contents do not vary with the relative contribution of chlorite and illite/sericite, but seem to have distinctly higher δ11B values in chlorite-rich samples. Loss of exchangeable Li starts during very low-grade metamorphism. Hence, δ7Li in sedimentary rocks with a significant relative amount of exchangeable Li is changed significantly, whereas δ7Li remains largely unaffected by very low-grade metamorphism in sedimentary rocks where most of the Li is structurally bound. At increasing metamorphic grade, the mobility of Li and B is largely controlled by the stability of muscovite/phengite and chlorite and whether metamorphically formed minerals sequester Li and B. However, overall loss of B and reduction of δ11B demonstrates that metamorphic tourmaline does no sequester released B quantitatively.The variation in the isotopic composition of Li and B during metamorphism is smaller (Li ∼10δ-units, B ∼8δ-units) than the isotopic variation in Palaeozoic shales from the former Gondwana shelf, where individual units have a relatively small variation (∼4–6δ-units) in δ7Li and δ11B, whereas the entire section – depending on weathering history and deposition environment – encompasses a range of more than ∼20δ-units in δ7Li and δ11B. Thus, inferences on the behavior of Li and B and their isotopic composition during metamorphism that are derived from the comparison of metamorphic rocks with an “average protolith“ may be misleading.

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