A Rb-Sr age and isotope study of the Ikertôq, Nordre Strømfjord, and Evighedsfjord shear belts, West Greenland - outline and preliminary results

Abstract

In the 1977 and 1978 field seasons, samples were collected from both the Ikertôq and Nordre Strømfjord shear belts and adjacent unsheared terrains. The samples represent the main rock-forming and shear zone-forming events in these two areas. In addition, samples for preliminary study were collected by S. B. Jensen and J. Allaart from sheared and unsheared gneisses in the Evighedsfjord region. Rb-Sr total rock analyses will be carried out on all samples. Suitably banded samples of both sheared and unsheared gneisses will be cut along compositionallayering to examine the Rb-Sr system on a small whole rock scale. The main Sr-bearing mineral phases of samples throughout the study area will also be analyzed. The purpose of this study is threefold. First, it will establish the geochronological frarnework of the main rock-forming and shear zone-forming events in this region. Second, it will help to clarify the thermo-tectonic history of the study area (especially in its first stages), mainly through the use of Rb-Sr mineral analyses. Two mineral age traverses will be produced, one north-south traverse along the coast between Agto and Evighedsfjord, and another east-west traverse from the coast 70 km inland along Nordre Strømfjord. The third aim of the study is to investigate the response of the Rb-Sr isotope system to intense deformation in the shear belts. This region lends itseif to such an investigation for two main reasons: (a) the age(s) of deformation ean be independently established using synto late-tectonic intrusive rocks; (b) the time period between the origin of the gneisses and the formation of the shear belts is long, from several hundred to 1000 m.y., and thus sufficient for significant isotopic heterogeneity to develop prior to deformation.