期刊
GEOCHIMICA ET COSMOCHIMICA ACTA
卷 128, 期 -, 页码 158-177出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2013.11.032
关键词
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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 delta Li-7 and delta B-11 during progressive metamorphism, but Li and B do not behave coherently. Additions from late fluids affect the B isotopic composition (shifting delta B-11 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 delta Li-7 range (-3.4 to +0.4), whereas samples with significant contributions of exchangeable Li have a broad range of delta Li-7 (-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 delta B-11 values in chlorite-rich samples. Loss of exchangeable Li starts during very low-grade metamorphism. Hence, delta Li-7 in sedimentary rocks with a significant relative amount of exchangeable Li is changed significantly, whereas delta Li-7 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 delta B-11 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 similar to 10 delta-units, B similar to 8 delta-units) than the isotopic variation in Palaeozoic shales from the former Gondwana shelf, where individual units have a relatively small variation (similar to 4-6 delta-units) in delta Li-7 and delta B-11, whereas the entire section - depending on weathering history and deposition environment - encompasses a range of more than similar to 20 delta-units in delta Li-7 and delta B-11. 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. (C) 2013 Elsevier Ltd. All rights reserved.
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