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

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 (δ⁷Li) 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±69μg/g, 2σ) is higher than that of granulite facies stronalites (8±6μg/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 δ⁷Li, 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 δ⁷Li values are comparable with those of other pelitic sediments and likely reflect the δ⁷Li of their protoliths. An anomalous kinzigitic sample, with the lowest δ⁷Li (-8.4) and the highest [Li] (120μg/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 δ⁷Li. 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±2.6μg/g (2σ) and are isotopically similar to the kinzigites (δ⁷Li=-1.0±0.2, 2σ); the latter formed by dehydration melting and have lower [Li] (3±2.6μg/g, 2σ) that is heavier (δ⁷Li=+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 3.0-4.2μg/g (average 3.6±1.2, 2σ) and variable δ⁷Li 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 δ⁷Li 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 8μg/g or less, and the concentration weighted δ⁷Li of this section of the lower continental crust is +1.0, which is similar to a previous estimate (8μg/g, +2.5).