Radiometric and petrographic investigations of basement gneisses from well 7120/12-2, Troms I area, offshore northern Norway

Core samples from the metamorphic basement from Troms I area, offshore northern Norway, consist of a fine-grained grey gneiss of granodioritic composition alternating with medium-grained granitic layers. Progressive deformation of the granitic layers produces an augen gneiss and eventually a banded blastomylonitic gneiss. The grade of metamorphism corresponds to amphibolite facies, but subsequent lower grade alteration with chloritization of biotite, sericitization of feldspar and carbonate veining has occasionally taken place.
Whole-rock isotopic analyses (Rb-Sr) give scattered data points which do not allow the calculations of a whole-rock age. However, the whole-rock data indicate a Precambrian age, as a model age of 1960 M.a. is obtained by assuming an initial 87Sr/86Sr ratio of 0.7050.
A Rb-Sr biotite - whole-rock age of 381 ± 7 M.a. agrees well with biotite ages from other segments of the Norwegian Caledonides. A K-fsp - whole-rock age of 553 ± 32 M.a. might indicate an early Caledonian (Finnmarkian) influence in the area.