Rb-Sr ages and U-Pb zircon ages have been obtained for several granitic gneisses and for porphyritic rocks of an associated volcano-sedimentary unit, formerly assumed to be equivalent to the Gaub Valley Formation, in the area of the Rostock massifs situated in the Southern Margin Zone of the Damara Belt. The isotopic systems of the rocks give relevant information about the time of formation, despite relatively poor Rb-Sr and U-Pb results attributed to alteration caused by Damaran metamorphism or later uplift and erosion. Rb-Sr whole-rock data from granitic gneisses give an age of 1049 ± 28 Ma with an initial 87Sr/86Sr of 0.7061 ± 0.0002, while their U-Pb zircon data indicate a maximum age around 1200 Ma for the time of crystallisation. U-Pb zircon data of porphyritic members of the metavolcanic unit define an age around 1100 Ma for the time of crystallisation. Rb-Sr whole-rock data from mafic members within this unit provide no information about the time of formation due to metamorphic disturbance. Rb-Sr mineral systems and Rb-Sr small-scale whole-rock systems were influenced by complete resetting during Damaran metamorphism around 530-490 Ma. In general, a period of granitoid emplacement and volcanic activity between 1200-1050 Ma is indicated by the results. These rocks are consequently considered to be equivalent to the Sinclair Sequence and related intrusives. It is concluded that the emplacement of the igneous rocks is related to collision-induced rifting, complementary to the Kibaran Orogenesis.