Geochemistry and petrogenesis of extrusive rocks, dykes and high-level plutonic rocks on the island of Oldra, Solund-Stavfjord Ophiolite Complex, western Norway

The island of Oldra, part of the Late Ordovician Solund-Stavfjord Ophiolite Complex of the western Norwegian Caledonides, comprises extrusive rocks, a sheeted dyke complex and high-level gabbros. The metabasalts are of N-MORB affinity, and their Nd isotopic composition ( k Nd = +7.8 to +8.4) indicates generation from a relatively homogeneous, but strongly depleted mantle. Eighty percent of the metabasalts classify as FeTi-basalts (FeO t /MgO >1.75, TiO 2 >2 wt%), but pseudostratigraphically their distribution varies significantly (51% of the extrusive rocks and 92% of the sheeted dykes). The contents of FeO t , MgO, Al 2 O 3 , Cr, Ni and Zr indicate that differences in depth of melting (30 to 12 kbar), crystal fractionation of olivine, plagioclase and clinopyroxene at different levels in the mantle and crust (<8 kbar) and magma mixing affected the composition of the rocks. The MgO-Zr relations of the established chemostratigraphy of the extrusive rocks indicate that magma mixing caused significant scatter among magma compositions.