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Continental Rifting |
1 Environmental Science Division, Lancaster University, Lancaster LA1 4YQ, UK
2 Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
The late Tertiary to Recent Kenya rift valley and the deeply dissected, Proterozoic, Gardar province of south Greenland are both parts of continental rift systems thousands of kilometres long. Both were generated within stabilized orogenic belts skirting cratonic forelands, in regions in which repeated reactivation of earlier structures has occurred. Both may represent failed attempts at continental separation.
Mafic magmatism in Kenya has ranged from strongly silica-undersaturated, melanephelinitic types through basanites to mildly alkaline and transitional basalts. Gardar activity was dominated by mildly alkaline and transitional basalts; basanites and nephelinites are rare or absent, the most undersaturated representatives being ultramafic lamprophyres. Very primitive (possibly primary) magmas are scarce in both rifts. Trace element chemistry suggests that the majority of the mafic rocks have lithospheric signatures, and that asthenospheric magmas had no direct access to the crust. The primary magmas were generated as small-degree melts in mantle sources which had experienced earlier depletion events and which were subsequently metasomatized. An important, mantle-derived feature of the Gardar basalts is their strongly aluminous nature, which appears to have generated an abundance of anorthositic bodies in the crust; this feature is not shown in the Kenya rift. Considerable amounts of underplating are inferred to characterize both rifts, limiting crustal thinning in regions of lithospheric extension.
Both rift zones have extensive axial dyke swarms, which are particularly focused on central complexes/caldera volcanoes. Kenya apparently lacks, however, the massive dykes, up to several hundred metres wide, of the Gardar province.
