Franz Josef Land (FJL) is a group of over 187 small islands covering an area of about 110.000 km2 and located between 44° 50´- 65°20´E longitude and 79° 55´- 81°51´N latitude. FJL consists of typical Continental Flood Basalts (CFB) with columnar appearence and a thickness of up to 620 m above sea level. According to the stratigraphy, the age of the basaltic vulcanism is approximately 140 Ma.The dominant rock type is tholeiitic basalt with Mg* between 44 and 57. However the basis of the central part of this province consists of tholeiitic andesites with Mg* between 29 and 38. The overlying sequences are typical tholeiitic basalts like everywhere in the province. We were not able to find rocks with MgO higher than 8.0 wt% even with intensive field work and intensive sampling (over 300 samples) A reasonable explanation is that over 65% of the surface are covered by ice.
Major element chemistry shows that FJL CFBs have undergone significant amounts of fractionation of olivine and pyroxene. Trends of increasing Al2O3 with decreasing MgO in the basaltic andesites indicate a minor plagioclase fractionation for this suite. The CaO/Al2O3 ratio correlated with Sc confirms that the basaltic andesites have undergone mainly clinopyroxene and less plagioclase fractionation. Furthermore the chondritic normalized REE do not show any negative Eu anomaly which would be the consequence of an extensive plagioclase fractionation.
Primitive-mantle-normalized trace element abundances show no negative Nb and Ta anomalies.Though both suites exhibit highly evolved magmas their Th/Ta ratios of about 2.1 are similar to that of the primitive mantle.
The eNd values vary from +3.8 to +7.2 and the Sr isotopes range from 0.70360 to 0.70580. In both cases the basaltic andesites show higher eNd values and lower Sr isotope values than the tholeiitic basalts. Therefore mixing of continental lithosphere with asthenospheric mantle or plume appears to be very unlikely. The low 87Sr/86Sr ratios which are not positively correlated with SiO2 provide additional evidences against a lithospheric contamination.
The FJL is a relatively small CFB province consisting of highly evolved magmas (tholeiitic basalts and basaltic andesites). However all geochemical signatures indicate that the entire FJL continental flood basalt province differs from the other known flood basalts because there are no evidences for a lithospheric mantle contribution. A crustal contamination, that is common to other CFBs, appears to be insignificant. Both suites kept their primitive signature (Th/Ta ratio is approximately 2.1). Basaltic andesites show evidences for high pressure fractionation. They probably do not have a common magma source with the tholeiitic basalts. However their magma sources according to their elemental and isotopic compositions should be very similar.