A mildly depleted upper mantle beneath southeast Norway: evidence from basalts in the Permo-Carboniferous Oslo Rift

Abstract

Major and trace element compositions and Rb-Sr and Sm-Nd relations are presented for a series of 290–300 Ma old basaltic lavas (B 1) from Vestfold and for the single B 1 flow at Krokskogen in the Permo-Carboniferous Oslo Rift. The Vestfold sequence consists of subalkaline to alkaline basalts, potassic trachybasalts and shoshonites. At Krokskogen B 1 is a subalkaline basalt. The Vestfold basaltic lavas show a relatively wide range in trace element concentrations, and Ta-enriched trace element abundance patterns. Ratios between pairs of incompatible elements show small, but significant variations e.g., Th/Ta: 1.2-1.8. The highest Th/Ta (2.1) is found in the Krokskogen basalt. A Th/Ta of 1.2 is typical of MORB and OIB, whereas the ratio 1.8 is typical of the lower crust. This variation in Th/Ta is not linked to trace element concentrations, but is accompanied by a regular shift in age-corrected ε Nd , ε Sr and 206Pb 204Pb from + 4.1, −10 and +19.2 to −1, +18 and +17.7, respectively. The observed variations in incompatible element ratios, initial isotopic ratios and trace element concentrations are interpreted as follows: The basaltic magmas which gave rise to the Vestfold and Krokskogen lavas originated in lithospheric mantle, which was mildly depleted with respect to Nd-Sr isotopic ratios but relatively rich in radiogenic Pb (PREMA-type). These magmas were retained in the lower crust where they crystallized ol + cpx; some of the magmas were also contaminated. The highest degree of contamination is seen in the lower part of the Vestfold sequence and in the Krokskogen basalt. At the present level of information about the Oslo Rift (no data are available for its southern, submerged part), rift magmatism appears to be dominated by a PREMA source in the pre-rift lower lithosphere. The dominance of a lithospheric mantle source in the rift magmatism suggests that the Oslo rifting event was initiated by passive mechanisms.