A Rb-Sr age study from the Mosjøen unit, Helgeland Nappe Complex, and its bearing on the timing of tectonometamorphic events within the Uppermost Allochthon, Central Scandinavian Caledonides, Norway

The Mosjøen unit within the Helgeland Nappe Complex is made up of a polydeformed supracrustal sequence consisting mainly of greenschists, greenstones and layers of calcite and dolomite marbles, which have been intruded by large masses of igneous rocks ranging in composition from gabbroic to granitic. The main structural elements and the metamorphic maximum are related to the D2-deformation. The Mosjøen gabbro, yielding an igneous Rb-Sr mineral isochron age of 420 ± 8 m.y. (I0 = . 70412 ± .00003), was emplaced during the D2 event. A 433 ± 11 m.y. total rock Rb-Sr isochron was obtained for a granitic dyke crosscutting the D2 structures. The geochronological data are interpreted to indicate a Silurian age for the main tectonometamorphic event (D2) in the Mosjøen unit.
Previous geochronological studies from other parts of the Uppermost AUochthon demonstrate strong early Caledonian (late Cambrian to early Ordovician) metamorphism and deformation prior to Silurian deformation. The present tectonostratigraphic sequence within the Uppermost Allochthon was established during the main Caledonian thrusting event in the late Silurian to early Devonian.