1 Department of Geoscience, Science and Technology, Aarhus University2 Universitet i Oslo3 University of Oslo4 Royal Holloway University London5 University London Imperial Coll Sci Technol & Med6 Department of Geoscience, Science and Technology, Aarhus University
This paper presents major, trace-elements, and Sr-Nd isotopes for two prominent sills formed during the opening of the North Atlantic, sampled by the Utgard borehole (6607/5-2) in the VOring Plateau. The Utgard sills are compared to opening-related lavas recovered from ODP Leg 104 Hole 642E farther west on the VOring Plateau and on the NE Greenland conjugate margin. The Utgard sills (3.6-5.9 wt % MgO) are enriched in strongly relative to moderately incompatible trace elements and have Sr-87/Sr-86 and Nd-143/Nd-144 ratios of 0.70380-70387 and 0.51292-0.51293, respectively, in the Upper Utgard Sill, and 0.70303-0.70306 and 0.51297-0.51299 in the Lower Sill. Alteration is minor. The Utgard melts originated by partial melting of an asthenospheric, depleted mantle source (DMM or Iceland Rift Zone, IRZ, type) with chemical characteristics similar to the source that gave rise to NE Greenland lavas. The Utgard magmas underwent extensive fractional crystallization in the lower crust (Upper Sill: >70%; Lower Sill: >55%) with removal mainly of olivine and pyroxenes, accompanied by 1% assimilation of crustal melts. This crystallization formed significant masses of dense cumulates (approximate to 3.25 g/cm(3)) (underplating). Assuming an areal extent similar to that of the two sills, we estimate a composite layer of ultramafic cumulates mixed with less dense country rocks to be >320 m thick beneath the two Utgard sills and >8.8 km beneath the thickest part of the VOring Plateau lavas. Opening-related cumulates may thus account for a significant part of the lower crustal high-velocity, high-density bodies (average density 3.1 g/cm(3)) along the Norwegian margin.