Continental Magmatism (Mantle-Derived) Usually associated with continental rifting Some in stable cratonic areas Magma-types Qtz Tholeiites, Tholeiites + Alkali Basalts Alkaline (Na 2 O+K 2 O+CaO>SiO 2 ), SiO 2 - undersaturated. Basanites, phonolites, tephrites. Peralkaline (Na 2 O+K 2 O>Al 2 O 3 ). These can be SiO 2 - undersaturated (nephelinites, leucitites, phonolites) or oversaturated (e.g. Peralkaline rhyolites, alkali granites) Ultrabasic (e.g. kimberlites, lamproites)
Continental Magmatism (Mantle-Derived) Doming and lithospheric thinning by ductile flow Large scale melting of lithosphere producing flood basalts
Flood Basalts Mg-contents 5-8% are too-low to be directly derived from the mantle Probably stored in magma chambers Also contain few phenocrysts
Flood Basalts Extrusion rates of up to 1 M m 3 s -1
Flood Basalts Columnar jointing in flood basalt
Flood Basalts Enrichments similar to E-morb Mainly fertile mantle. Some distinct depletions due to crystal fractionation. Not Mg-rich enough to be direct melts of the mantle. Accumulation in large magma chambers.
Continental Magmatism: Rift Stage Enriched in incompatibles relative to E-MORB. Similar enrichment to OIB = plume like enrichment.
Continental Magmatism: Rift Stage Some melting in lithosphere Some contamination by crust
Continental Magmatism: Afar Stage Qtz tholeiites, alkali basalts further from plume. N-MORB-like signatures
Extends 3,700 km Rifting began in the early Tertiary around the Ethiopian dome. Rifting in the Miocene in Kenya and Tanzania.
Pre-rifting Flood basalts Rifting-stage Alkaline volcanics Afar Alkali basalts and quartz tholeiites
Cratons: Kimberlites Potassic ultrabasic rocks dominated by xenocrysts of olivine, Cr-diopside, phlogopite etc. Matrices consist of serpentine, phlogopite(mica), carbonates, perovskite. More a clan of related rocks rather than specific rock types. Carbonate rich – basaltic kimberlites Mica rich - Orangeites