The age of the Precambrian basement in western Troms, Norway

Two granites in western Troms (Ersfjord granite and Rødberg granite), previously thought to be Caledonian intrusives, have been dated by the Rb-Sr whole rock method. A seven-point Rb-Sr isochron age of 1706±15 Ma and an initial 87Sr/86Sr ratio of 0.70404±0.00034 (1[sgrave]) was obtained on the Ersfjord granite. An isochron plot of seven samples from the Rødberg granite failed to define an isochron age, but also suggests a Precambrian age for this intrusive. On the basis of these ages it is concluded that no deep-seated reactivation of the Precambrian basement took place in western Troms during the Caledonian orogeny. The ages obtained correlate with the latest episode of mangerite intrusion in the Lofoten-Vesterålen area and with the late syn- and post-kinematic Svecokarelian granites in northern Sweden.