The major, trace element, and Li isotopic compositions of granodiorite and metapelites from the Onawa pluton and surrounding contact aureole have been measured to document the behavior of Li and its isotopes during progressive metamorphic dehydration. Major and trace element concentrations in metapelites of the contact aureole change little, while loss on ignition (LOI) decreases with increasing metamorphic grade, which ranges from regional chlorite-zone metamorphism far removed from the pluton to partially melted rocks adjacent to the pluton. Lithium concentrations in metapelites from all zones correlate with LOI and abundances of the main Li-bearing minerals, chlorite + biotite + muscovite, decreasing by a factor of two (from 130 ppm far removed from the pluton to 64 ppm adjacent to the pluton). In contrast, Li isotopic compositions remain relatively unchanged across the aureole (δ7Li = − 3.5 to + 1.0), and are comparable to the range observed in schists and unmetamorphosed shales. Compared to the surrounding metasediments, the granodiorite has a lower Li content (45 ppm) and a comparable δ7Li value of − 0.2, similar to those of other granites and the average upper crust (0 ± 2).