A review of the distinctive spatial and temporal compositions of basalt sequences associated with slow-spreading zones influenced by mantle plumes and their petrogenetic implications

Abstract

Systematic and substantial temporal and local spatial variations in trace element and radiogenic isotope ratios are reviewed in basalts sequences from East Greenland, Iceland, Faeroe Islands, Rockall Trough, Hebridean Islands and the Mohns Ridge. These variations are indicative of local scale mantle heterogeneities relating to mantle plume influences in these zones of incipient rifting and slow-spreading extensional tectonics. The magnitude and range of key elemental and isotopic variations are reviewed and found to be similar in each region. To effectively characterize the compositional variations, two relevant reference compositions are employed: worldwide averages for mid-ocean ridge basalt (N-MORB) and intraplate basalt (WLD). Profiles of regional average trace element concentrations and Pb-isotopes normalized to the WLD reference also reveal characteristic patterns. Some basalt series from East Greenland, Faeroe Islands, Rockall Trough and the Hebridean Islands have been affected by crustal contamination. However, the trace element ratios and normalized profiles are still able to reveal mantle heterogeneities influencing those lava sequences. Based on the regional comparisons in spatial and temporal variations, dominated by fertile compositions in the earlier stages and more refractory compositions in the later stages, a conceptual petrogenetic model is developed and illustrated. This incorporates pulsed-plume diapirism and sequential, incremental upper mantle dynamic partial melting forming distinctive magma pulses, Asthenosphere outflow and magma mixing in an interconnected network of lower crustal, sill-like magma chambers are additional features of the model. This review reveals that in slow-spreading or incipient-rifting, extensional regions, where mantle plumes are laterally constrained for longer periods than at fast spreading ridges, a distinctive compositional evolution occurs in the basalt sequences generated.