Earth's oldest well-preserved mafic dyke swarms in the vicinity of the Isua greenstone belt, southern West Greenland

The Isukasia region of southern West Greenland contains the Earth’s oldest known supracrustal rocks (the c. 3.8‐3.7 Ga Isua greenstone belt: Appel et al. 1998) and well-preserved mafic dyke swarms (Inaluk and Tarssartoq dykes: Nutman et al. 1983). This report describes field investigations of the dykes carried out in 1999 as part of the Isua Multidisciplinary Research Project, and summarises current knowledge of the dykes. The project was initiated in 1997 with the aim of coordinating a detailed reinvestigation of this geologically important region. Fifteen members of the group were involved in field work in 1999 and this article represents only one aspect of the work. The occurrence, state of deformation and geochemical composition of the dykes provide important information on the evolution of the continental crust into which the dykes were intruded. Cross-cutting relationships between dykes and host rocks, and between different generations of dykes, yield valuable relative time constraints, and dykes provide significant time markers between structural and metamorphic events. The compositions of the least altered and least metamorphosed dykes help to define the properties of their mantle source region, as well as the nature of the crust through which the magmas travelled.