Radiometric dating of the Jungfrun granite

Abstract

The Jungfrun massif consists of a homogeneous alkali-feldspar granite with scattered light aplitic inclusions of unknown genesis. Dating of the Jungfrun massif shows an inconsistent pattern when comparing the K-Ar, Rb-Sr and U-Pb systems. A Rb-Sr whole-rock isochron age of 1386 ± 21 Ma is accepted as representing the time of intrusion while a U-Pb discordia age of 1480⁺¹⁰ _-31 Ma on zircons is dismissed as erroneous. The reason for this is presence of cores in the zircons. A K-Ar age of 1412 ± 20 Ma on biotite points to excess Ar, probably inherited from a former country rock. The small anorogenic massifs of Jungfrun and the related Götemar granite are not unique as earlier assumed. In southeastern Baltoscandia other anorogenic granites formed 1400 ± 50 Ma ago are found. These granitoids, collectively known as the Karlshamn group, show a progressive evolution from calc-alkaline character towards somewhat more alkaline types with time. The latter seem to be related with the Jungfrun-Götemar alkali-feldspar granites chemically and in age. This group of younger granites also show similarities in their Ti, Zr, Nb and Y ratios. Hence, it is tentatively suggested that the anorogenic granites in southeastern Baltoscandia have a common origin.