Rb-Sr whole rock 'ages' from reworked basement gneisses in the Umanak area, central West Greenland

This note reports the results of a Rb-Sr whole rock study of two of the main gneiss units in the southern part of the Rinkian mobile belt, central West Greenland. These gneisses belong to the basement upon which the metasedimentary formations of the Karrat Group were deposited. One of these formations, the Marmorilik marble formation, can be traced from its type locality where a primary unconformity at the base of the formation is preserved, into the southern part of the Umanak area where both the marble and the basement gneisses have been involved in at least two phases of Rinkian recumbent folding. Both the gneiss units studied were sampled in areas where demonstrably post-Marmorilik Formation (i.e. Rinkian) recumbent folding is conspicuous. The one gneiss unit is a granodioritic biotite gneiss which is the main gneiss type in the Umanak area. Samples of this collected within an area of 50 m2 yielded an isochron age of 2629 ± 246 Ma, Sr; 0.7033 ± 0.0010 (2o errors). The other gneiss studied is granodioritic augen gneiss known from earlier dating in an area of low Rinkian deformation to be at least 2500 Ma old. Samples of this gneiss collected in an area of very strong Rinkian deformation did not give an isochron but scatter around a 2500 Ma reference line. Although this study has not yielded accurate ages for the gneiss units investigated, it has at least pro­vided further confirmation that the basement gneisses of the Umanak area are Archaean. Furthermore, the results show that recumbent folding and strong deformation under amphibolite facies conditions can disturb but will not necessarily reset Rb-Sr whole rock isochrons - an observation that accords with results from the gneissic cores of the Pennine nappes in the Alps.