GRENVILLIAN U-PB ZIRCON AGES OF QUARTZ PORPHYRY AND RHYOLITE CLASTS IN A METACONGLOMERATE AT VIMSODDEN, SOUTHWESTERN SPITSBERGEN

Proterozoic metasupracrustal rocks form a NNW-SSE trending basement zo ne along the western coast of Spitsbergen. The rocks show complex stru ctures as a result of both Caledonian and Tertiary deformation, and mo st of the subordinate metaigneous rocks are not suitable for isotopic age determination. Some zircon-bearing rocks were found in the southwe stern part of Spitsbergen and an attempt of U-Pb dating was performed. U-Pb dating was carried out on zircon fractions from quartz porphyry and rhyolite clasts in a metaconglomerate unit of the Pyttholmen Forma tion northwest of Hornsund, southwestern Spitsbergen. The Pyttholmen F ormation is considered to be a lateral equivalent of the upper part of the Gulliksenfjellet quartzite and in the same time as the upper part of the Skilijellet metavolcanites. Therefore, the obtained ages are a pplicable to the age of the Skalfjellet igneous activities. Some of th e dated samples are strongly schistose and their magmatic origin is di fficult to confirm; the interpretation of the isotopic results is not well constrained; however, some explanations are possible which refer to the known geological conditions; an igneous age of siliceous volcan ic rocks of ca. 1200 Ma, inherited zircon ages of ca. 2500 Ma and a re gional metamorphic age of ca. 930 Ma. The last age belongs to the Gren villian period and is conformable with the Rb/Sr whole rock age obtain ed from the garnet-biotite schists of the Isbjornhamna Group underlyin g the Skalijellet metavolcanites.