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Elsevier, Chemical Geology, 1-2(289), p. 76-85

DOI: 10.1016/j.chemgeo.2011.07.014

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The behavior of lithium in amphibolite-to granulite-facies rocks of the Ivrea–Verbano Zone, NW Italy

Journal article published in 2011 by Lin Qiu, Roberta L. Rudnick ORCID, William F. Mcdonough ORCID, Fernando Bea
This paper is available in a repository.
This paper is available in a repository.

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Abstract

To investigate the behavior of Li during high-grade metamorphism and the Li composition of the lower continental crust, the Li concentrations ([Li]) and isotopic compositions (δ 7 Li) of amphibolite-to granulite-facies metapelites, leucosomes and metabasites from the Ivrea–Verbano Zone, NW, Italy were determined. The average [Li] of amphibolite facies kinzigites (79 ± 69 μg/g, 2σ) is higher than that of granulite facies stronalites (8 ± 6 μg/g, 2σ) that experienced partial melting via biotite dehydration reactions. Biotite abundance and the [Li] in metapelites correlate positively, reflecting the importance of Mg-bearing phyllosilicates in controlling the Li budget of the metapelites. Despite the loss of Li following biotite breakdown, there is no significant change in δ 7 Li, which averages − 1.4 ± 2.0 (2σ, excluding an anomalously light sample) in the kinzigites (amphibolite facies) and + 0.9 ± 2.9 (2σ) in the stronalites (granulite facies). Both average δ 7 Li values are comparable with those of other pelitic sediments and likely reflect the δ 7 Li of their protoliths. An anomalous kinzigitic sample, with the lowest δ 7 Li (− 8.4) and the highest [Li] (120 μg/g), has extremely low Ba and Sr concentrations and the highest CIA value, all indicators of a highly weathered protolith, which was likely isotopically light prior to significant metamorphism. This sample may also have experienced Li addition via diffusion, increasing [Li] and lowering δ 7 Li. The leucosomes in the kinzigites and stronalites are interpreted to have formed in different ways; the former precipitated from hydrothermal fluids, have an average [Li] = 13 ± 2.6 μg/g (2σ) and are isotopically similar to the kinzigites (δ 7 Li = − 1.0 ± 0.2, 2σ); the latter formed by dehydration melting and have lower [Li] (3 ± 2.6 μg/g, 2σ) that is heavier (δ 7 Li = + 6.0 ± 6.3, 2σ) than that of the stronalites or kinzigites. The mineralogical and compositional characteristics of the stronalites are consistent with substantial Li loss during partial melting. A melting model indicates that Li removed from the stronalites may contribute to the formation of S-type granites or Li-enriched pegmatites. Three granulite-facies metabasites have relatively constant [Li] of 3.0–4.2 μg/g (average 3.6 ± 1.2, 2σ) and variable δ 7 Li of −3.2 to + 3.3 (average 0 ± 6.5, 2σ). While these concentrations are typical of those of possible basaltic protoliths, the highly variable δ 7 Li suggests Li mobility during metamorphism. Collectively, the Li signatures in granulite facies stronalites and metabasites indicate the [Li] of this section of lower continental crust is close to 8 μg/g or less, and the concentration weighted δ 7 Li of this section of the lower continental crust is + 1.0, which is similar to a previous estimate (8 μg/g, + 2.5). Published by Elsevier B.V.