Paleoproterozoic mafic dike swarms of northeast Laurentia: products of plumes or ambient mantle?

The occurrence of mafic dike swarms in Archean and Paleoproterozoic terranes is often cited as evidence for mantle plume activity associated with abnormally high mantle potential temperatures (Tp). Quantitative modeling of rare-earth-element (REE) abundances for the 2.04 Ga Kangâmiut dike swarm in west Greenland and comparisons among other Paleoproterozoic dike swarms in northeast Laurentia indicates that the conditions for mantle melt generation were similar beneath much of Laurentia and only ca. 120 °C hotter than present-day ambient mantle (Tp ca. 1300 °C). These thermal conditions correspond closely to estimates of ambient mantle potential temperatures during the Paleoproterozoic era based on secular cooling models and continental freeboard constraints. Thus, the geochemistry of most Paleoproterozoic dikes of northeast Laurentia can be accounted for by partial melting of upwelling mantle accompanying passive rifting and do not necessitate plume activity. Ambient mantle conditions during the Paleoproterozoic (Tp ca. 1420 °C) are similar to those inferred for the Paleogene volcanic rift margins of the North Atlantic igneous province, where the rate of crustal production associated with early oceanic spreading was two to three times higher than at the modern ridge. Likewise, rifting under ambient mantle conditions in the Precambrian could have resulted in similar rates of crustal productivity and thus contributed significantly to continental growth.