Noritic dykes of southern West Greenland: early Proterozoic boninitic magmatism

Abstract

Two major swarms of early Proterozoic (ca. 2.1 Ga) basic dykes occur within the Archaean craton of southern West Greenland. One swarm comprises ophitic and sub-ophitic tholeiitic dolerites, while the other (the BN dyke swarm) constitutes mainly norites in which pyroxenes and olivine are enclosed by plagioclase oikocrysts. The close geochemical similarity between a quenched norite and the coarser-grained varieties indicates that the composition of the latter type has not been significantly modified by crystal accumulation. The BN dykes are geochemically distinctive, most having high MgO (ca. 16%), Cr and Ni contents in conjunction with relatively high SiO2, light rare-earth (REE) and large ion lithophile (LIL) element concentrations. The texture, mineral chemistry and petrochemistry of the quenched noritic dyke all bear strong resemblances to those features in modern boninitic lavas. The BN dykes also correspond to proposed parental liquids of the Bushveld Complex and other major layered basic igneous intrusions. The two dyke swarms are petrogenetically distinct. The tholeiitic dolerites were derived from a relatively undepleted, primordial mantle while the noritic dykes originated from a metasomatized harzburgitic source. The wide-spread distribution of similar Proterozoic intrusions suggests crustal underplating by harzburgitic mantle on a world-wide scale at this time.