Late Vendian U-Pb zircon age of a dolerite dyke from near Hamningberg, Varanger Peninsula - a preliminary report
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A dolerite dyke cutting very low-grade metasandstones of the Løkvikfjellet group near Hamningsberg, NE Varanger Peninsula, has yielded a U-Pb zircon age (upper intercept) of 554+20\/-14 Ma; and a lower intercept of c.375 Ma. This dyke is one of several in this area that have previously given K-Ar dates of c.360 Ma. These dykes are similar in field appearance, petrography and chemistry to dolerite dykes cutting Baikalian (Late Vendian) NW-SE-trending folds and cleavage on the Rybachi Peninsula, NW Russia; and all trend between NE-SW and c.N-S. It is suggested that the Hamningsberg dyke, and the other, comparable dykes with published Late Devonian K-Ar 'ages', probably intruded along 'ac' extensional, master fractures either during the terminal stages of, or shortly after the Baikalian deformation in Late Vendian time. The lower intercept date of c.375 Ma is interpreted as reflecting a Mid Devonian isotopic disturbance related to a Caledonide (Scandian) tectono- thermal event.