The oxygen isotope composition of Hercynian granites and pre-Hercynian gneisses from the Schwarzwald,SW Germany

The oxygen isotope composition of 56 Hercynian granites and 42 pre-Hercynian gneisses has been investigated. In addition some mineral δ ¹⁸O data and 5 δD values of whole rocks have been obtained. The granites from the N-Schwarzwald show, in general, relatively uniform δ ¹⁸O values between 11.5 and 13.5‰, those from the S-Schwarzwald are less uniform and lighter in ¹⁸O and range from 2.3 to 11.5‰. The gneisses from the pre-Hercynian basement exhibit more or less the same variation and range from 1.7 to 10.4‰. δ ¹⁸O values <6‰ only occur in the S-Schwarzwald and obviously indicate hydrothermal interactions of meteoric waters, which probably took place after the emplacement and solidification of the granites and which equally affected granites and gneisses. Due to the nearly identical ¹⁸O/¹⁶O ratios of S-Schwarzwald granites and gneisses, it is proposed that such gneisses in the pre-Hercynian basement qualify as the precursor rocks of the S-Schwarzwald, granites whereas for the N-Schwarzwald granites crustal rocks with heavier δ ¹⁸O values, unknown from the present surface, have to be postulated. This distribution is also reflected on a ⁸⁷Sr/⁸⁶Sr-¹⁸O/¹⁶O diagram.