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A late Neoproterozoic eastern Laurentian superplume: Location, size, chemical composition, and environmental impact

Department of Geological Sciences, Rutgers University, Newark, New Jersey 07102

A database consisting of 25 data-sets has been compiled that includes published chemical analyses of most known eastern North American occurrences of late Neoproterozoic (Vendian) flood basalt and dike swarms together with some new geochemical data. Four additional eastern North American basalt occurrences of probable early Paleozoic age were also examined. The Vendian occurrences comprise the Central Iapetus magmatic event of Eastern North America which extends from the Long Range dikes of Labrador and Newfoundland to the Catoctin flood basalts of Virginia and includes the Grenville dike swarm of Ontario and Quebec. The strike of the Vendian dike swarms converge near a major gravity and magnetic anomaly at Sutton Mountain, Quebec. The chemical composition of the Vendian basalts and diabase dikes plotted onto spider diagrams normalized to the composition of the silicate earth can be subdivided into a tight cluster of parallel lines displaying HFSE enrichment and a more diverse group that is less enriched. The tight grouping of the enriched cluster suggests a mutual genetic relationship consistent with mantle plume derivation based largely on chemical similarity to standard Ocean Island Basalt (OIB). However, detailed examination of this eastern Laurentian OIB related cluster (LOIB) reveals regional variations in chemical composition, particularly TiO₂ and Zr, that can be contoured to delineate a narrow lens shape with peak concentrations centered close to the Sutton Mountain triple junction. The central portion of the LOIB is geochemically the same as superplume derived basalt. Radiometric evidence indicates that the LOIB superplume magmatic activity peaked at about 550 Ma.

The less enriched group geochemically resembles some of Earth’s larger continental flood basalts, such as the Columbia River basalt province, derived from a subcontinental lithospheric mantle source mixed with magma from a mantle plume source. Radiometric and paleomagnetic evidence indicates that this less enriched group is about 615 to 564 Ma and probably extruded out of rifts that preceded the break-up of Pannotia. This mid-Vendian magmatism is consistent with the early stages of deep mantle plume upwelling but may be genetically independent of the superplume magmatism that followed.

The four early Paleozoic basalt suites examined are interpreted as post-LOIB, Iapetan Ocean ridge basalt accreted onto eastern Laurentian or perhaps basalt extruded from transform faults that intersected the trailing edge of Laurentia.

If LOIB magmatism was generated by a superplume head, there may be important environmental implications. The LOIB event may have been partially responsible for considerable mantle outgassing, particularly carbon dioxide and nutrients, global warming, and major increases in the growth rate and deposition of marine life that occurred during the early Cambrian.

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