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).
- Amphibolites-facies and granulite-facies metamorphism
- Ivrea-Verbano Zone
- Partial melting