Age and tectonic setting of the Tysfjord gneiss granite, Efjord, North Norway

The TYsfjord gneiss granite, forming an easterly salient from the Lofoten-Tysfjord-Vesterålen Basement
Region, is dated at 1742 ± 46 Ma by the Rb-Sr whole-rock method. lts 87Sr/86Sr initial ratio of
0. 71151 ± 0. 00247 is distinctly higher than the initial ratios of the plutons of the Rombak Window as
well as the mangeritic rocks of the Hamarøy and Lofoten areas, suggesting that no simple comagmatic
relationships exist between these plutons. A tentative model assumes the Tysfjord granite to be derived
by partial melting in the lower to middle crust associated with the emplacement of the slightly older mantle(?)-derived mangerites. Much of the foliation and lineation in the Tysfjord granite are considered to be Caledonian fabric elements formed early in the orogenic cycle, most probably associated with an early phase of nappe emplacement. The Caledonian structures are not restricted to the uppermost part of the basement, but occur at least 2500 metres structurally below the basement/cover contact. Evidence from outside the Tysfjord area suggests that the nappe emplacement, and thus the fabric elements, cannot be older than 410 Ma. Two biotite-whole rock isochron ages from the Tysfjord gneiss granite, of 367 ± 8 Ma and 347 ± 7 Ma respectively, are interpreted as late Caledonian cooling ages associated with uplift of the Tysfjord basement above the blocking temperature (350°C) for diffusion of Sr in biotite.