GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF THE CONTINENTAL FLOOD BASALT MAGMATISM IN FRANZ JOSEF LAND, ARCTIC RUSSIA

Abstract

The Continental Flood Basalts in Franz Joseph Land, Russia, consist of basaltic andesites and tholeiitic basalts with basaltic andesites always underlying the tholeiitic basalts. Both lava types lack negative Nb-Ta anomalies precluding any interaction 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 requires for basaltic andesites 11% melting of a plume at a pressure of 45 kbar. The starting picrite with 20 wt% MgO became less Mg-rich and achieved a composition of 11 wt% MgO mainly by olivine fractionation. The subsequent evolution of these 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.