<p>Trace elements and isotopes in mineralized veins are a powerful tool to reconstruct fluid sources and conditions of vein formation. However, while the isotopic composition of mineral veins has been intensively studied, there is comparatively little information on the isotopic composition and evolution of the rocks hosting the mineral veins. Here, we investigate Sr-isotopes of metamorphosed and deformed marly sediments from a fossil accretionary wedge (Glarus Alps, Switzerland) and match our findings against published and new isotope data from mineral veins from the same study area. We obtain the Sr-isotopic composition of calcite from the metamarls applying a sequential leaching method, which minimizes the contribution of radiogenic Sr from silicates. Moreover, correlating the Rb content and the Sr-isotope values of the leachates indicates mixing of Sr sources and allows us to correct for the small but relevant radiogenic Sr contribution from the silicates. The corrected ⁸⁷Sr/⁸⁶Sr ratios of the matrix calcites vary significantly between different samples and show a covariation with the bulk silicate content of the host rock. Given the covariation, we propose that matrix calcite Sr-isotope composition depends on the calcite/silicate ratio and the amount of Sr exchange between minerals during recrystallization. Similar Sr-isotopic compositions between most analyzed matrix–vein calcite pairs show the connected evolution between matrix and vein isotope data, which indicates local mass transfer from the matrix into the vein.</p>