Coarse-grained granitic rocks underlie the Caledonian series in the Sjangeli-Rombak window, North Norway. A Rb-Sr whole-rock isochron on these granites gives a date of about 1780 Ma. The initial ⁸⁷Sr/⁸⁶Sr ratio of 0.700 + 0.006 suggests that the granites were derived from the mantle or from an isotopically similar region without significant incorporation of sialic material. The distribution of pubIished initial ratios for crystalline rocks in North Norway and Sweden dated between 1845 and 1535 Ma is consistent with the existence of an eastward-dipping NW -SE trending subduction zone beneath the area during this period.