Sources of Proterozoic mafic dyke swarms: Constraints from Th/Ta and La/Yb ratios

Th/Ta and La/Yb ratios in mafic dykes provide an important constraint on their magma sources and magma evolution. These element ratios clearly indicate at least three sources must be available for some swarms and that few swarms can be accounted far by only one source. Some dyke magmas appear to reflect mixing of mantle plume sources with Ta-depleted Archean subcontinental lithosphere. Systematic secular compositional changes in Th/Ta or La/Yb ratios are not observed in dyke swarms ranging in age from 2.45 to 1.14 Ga from the southern Superior Province, and in the giant Mackenzie swarm (1.27 Ga) there is no change in either element ratio as a function of distance from an inferred plume source. Paleoproterozoic dyke swarms range from those dykes with low Th/Ta and La/Yb ratios, indicating a strong contribution of depleted or primitive mantle, probably in a plume source, to picrite/norite dykes with very high ratios, perhaps indicative of a source in the Archean subcontinental lithosphere. There is an overall shift in composition of dykes from high Th/Ta ratios in the Paleoproterozoic to low ratios in the Neoproterozoic, reflecting a decrease in importance of Archean subcontinental lithospheric sources and an increase in importance of plumes containing enriched mantle components (recycled sediments and oceanic lithosphere). The delayed appearance of enriched mantle components in dyke sources until the Neoproterozoic may reflect the time it takes to recycle oceanic lithosphere through the lower mantle, beginning in the Late Archean.