Dating of late Archaean crustal mobilisation north of Qugssuk, Godthåbsfjord, southern West Greenland

Abstract

The Taserssuaq tonalite, which is slightly younger than or coeval with the common grey gneisses north of Godthåbsfjord, has yielded a zircon U-Pb age of 2982 ± 7 Ma, and an apparent Rb-Sr whole-rock age of 2882 ± 36 Ma (MSWD = 1.57, initial 87Sr/86Sr = 0.7017). The minerals were isotopicaIly equilibrated at 2500 Ma, and finally biotite was reset at 1700 Ma. The Qugssuk granite, an adjacent granitic mobilisate, has yielded a Rb-Sr age of 2969 ± 32 Ma (MSWD = 1.09, initial 87Sr/86Sr = 0.7020). The intrusion of the Taserssuaq tonalite is probably dated by its zircon age, which broadly correlates it with the Nilk gneisses. Field relations and microtextures strongly suggest that the Qugssuk granite is younger than the Taserssuaq tonalite and post-dates granulite facies metamorphism in the area, and its formation may be related to the extensive retrogression of the Taserssuaq tonalite. Isotopic data may support this interpretation in spite of the apparent inconsistencies in the age values.