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

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 7Li) 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 7Li, 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 7Li values are comparable with those of other pelitic sediments and likely reflect the δ 7Li of their protoliths. An anomalous kinzigitic sample, with the lowest δ 7Li (− 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 7Li. 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 7Li = − 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 7Li = + 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 7Li 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 7Li 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 7Li of this section of the lower continental crust is + 1.0, which is similar to a previous estimate (8 μg/g, + 2.5).