Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low delta Li-7 values (-6.4 parts per thousand to +2.4 parts per thousand, with an average value of -2.1 parts per thousand) in comparison to the country rock metagranites (delta Li-7 = -1.2 parts per thousand to +3.6 parts per thousand, with an average value of +1.4 parts per thousand). The delta Li-7 values of whiteschists do not show any correlations with major elements (e.g. Mg, Fe), Li, or isotopes (i.e., delta Mg-26 and delta O-18 values) and cannot be explained by kinetic fractionation. Hence, the low delta Li-7 values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low delta Li-7 values (down to -4 parts per thousand) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low delta Li-7 fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior. (C) 2019 Elsevier B.V. All rights reserved.