Mantle Source Heterogeneity in the North Atlantic Igneous Province and the Iapetus Connection

The prevailing paradigm for the early Paleogene North Atlantic igneous province holds that the large volumes of basalt erupted during continental breakup were generated by decompression of the ancestral Iceland plume arriving at the base of Pangea ~60 mys ago. Based on a comparative study of the geochemistry of early Paleogene basalts from the Greenland margins, an alternative model is explored. In this model, mantle sources tapped by partial melting reflect ambient upper mantle conditions at that time, specifically lateral differences in sublithospheric mantle composition coupled with modest temperature anomalies (<100 K). Basalts from central east Greenland are dominated by tholeiites enriched in iron and titanium, strongly fractionated rare earth element ratios, low Zr/Nb ratios, and radiogenic isotope compositions corresponding to immature HIMU values. These characteristics are not shared by early Paleogene basalts erupted in southeast and west Greenland, which are instead low in titanium, depleted in incompatible trace elements and possess radiogenic isotope compositions similar to depleted OIB and MORB. Isotopic relations for basalts from central east Greenland indicate a mantle source with a mean age of ~600 Ma, while southeast and west Greenland basalts are derived from significantly older mantle (>2 Ga). Such regional differences are not readily explained by current plume models, but correspond well to the basement geology of the region. It is deemed significant that the southwestern boundary of the Caledonian front and zone of closure of the Iapetus ocean correspond closely to the southwestern limit of the geochemical anomaly associated with the central east Greenland volcanic province. The distinctive basalt geochemistry and high melt productivity accompanying rifting within the Caledonian suture zone are attributed to the presence of subducted Iapetus crust in the upper mantle along this portion of the rifted margin. Given the depleted nature of basalts from southeast and west Greenland, similar "recycled" material apparently was not available south and west of the Caledonian suture. Likewise, comparisons between central east Greenland and modern Iceland basalts suggest that source heterogeneities north of the Caledonian front have changed through time. The connection between subduction processes involved in supercontinent assembly and mantle heterogeneities sampled during breakup reduces the necessity to introduce recycled material via a lower mantle plume early in the development of the North Atlantic ocean basin.