TIMING OF LOWER CRUST GENERATION IN THE VOYKAR OPHIOLITE MASSIF, POLAR URALS, RUSSIA: U-Pb (LA-ICP-MS) DATA FROM PLAGIOGRANITE ZIRCONS

The Uralide Orogen is one of the main Paleozoic belts formed during the assembly of Pangea. The present-day Urals (Uralian Mountains, N-S-trending through northern Eurasia) expose the relics of this Paleozoic orogen. The Voykar Massif, located at the Polar Urals, is an ophiolite assemblage composed mainly of mantle tectonite, gabbroic to ultramafic plutonic sequence and sheeted dikes with a MORB-type source. Geochronological data for this region show a wide range of ages and are mainly based on U-Pb zircon data, although some Rb-Sr and Sm-Nd are available. We present high-resolution U-Pb (LA-ICP-MS) analyses performed on zircon crystals extracted from a plagiogranite vein associated with a porphyritic dolerite dike. Magmatic zircon grains yielded a U-Pb age of 428.1±7.4 Ma, corresponding to the Wenlock Period (middle Silurian). This age suggests a late stage of oceanic crust generation in the Voykar basin. Different ages from plagiogranite and gabbroic bodies suggest that oceanic spreading lasted from ca. 490 to 425 Ma, i.e., at least 65 Ma or even longer, in the Voykar basin.