[en] Previous isotopic investigations of initial and younger, cratonal magmatic rocks have been completed by Sr studies of cratonal magmatic rocks. Both the delta ¹⁸O values and the ⁸⁷Sr/⁸⁶Sr ratios vary much more for initial magmatic rocks than for cratonal magmatic rocks. An isochron from 4 initial magmatic rocks from the Harz (3 kalispilites, 1 microsyenite) of an age of 360 millions of years and with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7034 shows that these four magmatic rocks originate unadulteratedly from the mantle. The initial ⁸⁷Sr/⁸⁶Sr ratios of the younger magmatic rocks and phonolites are in accordance with those suggested for the mantle so that these rocks can be considered as mantle derivatives. An age of 40.3 +- 1.4 millions of years was found for one phonolite. Possible mantle isochrons of initial rocks are discussed