Grenvillian U-Pb zircon ages of quartz porphyry and rhyolite clasts in a metaconglomerate at Vimsodden, southwestern Spitsbergen

Abstract

Proterozoic metasupracrustal rocks form a NNW-SSE trending basement zone along the western coast of Spitsbergen. The rocks show complex structures as a result of both Caledonian and Tertiary deformation, and most of the subordinate metaigneous rocks are not suitable for isotopic age determination. Some zircon-bearing rocks were found in the southwestern 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 Formation northwest of Hornsund, southwestern Spitsbergen. The Pyttholmen Formation 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 Skålfjellet metavolcanites. Therefore, the obtained ages are applicable to the age of the Skålfjellet igneous activities. Some of the dated samples are strongly schistose and their magmatic origin is difficult 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 volcanic rocks of ca. 1200 Ma, inherited zircon ages of ca. 2500 Ma and a regional metamorphic age of ca. 930 Ma. The last age belongs to the Grenvillian period and is conformable with the Rb/Sr whole rock age obtained from the garnet-biotite schists of the Isbjernhamna Group underlying the Skålfjellet metavolcanites.