REE and ¹⁸O/¹⁶O distributions in altered Variscan granites of the western Harz, Germany, and southern Sardinia, Italy.

Studies on whole-rock samples and mineral separates from granitic rocks of the Harz mountains and southern Sardinia have revealed post-magmatic changes of REE concentrations and O isotopic composition. The REE distribution patterns of the strongly altered rocks do not deviate significantly from the less-altered samples, but the REE patterns of the feldspars and biotites show characteristic changes. Some of the feldspars lost their typical positive Eu anomalies, whereas some of the biotites gained negative Ce anomalies. In the altered samples the quartz-feldspar O isotopic fractionation is either too low or even reversed relative to primary magmatic pairs. The observed effects are discussed, as the interactions of the cooling granitic rocks with either surface or deep-seated waters. It is concluded that high-T water-rock interactions with mixed formational metamorphic waters best explain the observations.-J.M.H.