Geochemical constraints on the origin of the Continental Flood Basalt magmatism in Franz Josef Land, Arctic Russia

The Continental Flood Basalts in Franz Joseph Land, Russia, consist of basaltic andesites and tholeiitic basalts with basalticandesitesalwaysunderlyingthetholeiiticbasalts.BothlavatypeslacknegativeNb-Taanomaliesprecludinganyinteraction or assimilation of continental crust. Ratios of highly incompatible elements like Th/Ce are similar to those of the oceanic island basalts and positive Nd values lying between MORB and Bulk Earth, indicate that the erupted magmas originated from an asthenospheric mantle plume. Major and trace element abundances that used to reconstruct the hypothetical parental magmas, indicate that for basaltic andesites this hypothetical magma originated by partial melting of an ascending plume at higher pressures than those for tholeiitic basalts. Our model requiresfor basalticandesites11% melting of aplumeat apressure of 45 kbar.Thestarting picrite with 20wt% MgO becamelessMg-richandachievedacompositionof11wt%MgOmainlybyolivinefractionation.Thesubsequentevolutionofthese lavas is characterised by fractionation processes in which clinopyroxene was the dominant phase. For tholeiitic basalts a starting picrite with 16 wt% MgO requires 23% melting of an ascending plume at a pressure of 30 kbar.