Major, trace, rare-earth element geochemistry and isotopic data have been obtained for offshore igneous rocks from around the northern Rockall Trough, NE Atlantic Ocean. Most comprise basalts derived from the sub Rockall-Hebridean asthenospheric mantle, but subcontinental lithospheric mantle generated the phono-tephrites of Rosemary Bank, and melting of upper crust was responsible for the peraluminous dacites of well 163/6-1A. Crustal contamination of the lavas around the Rockall Trough suggests that the Lewisian-Islay (Rockall Bank) basement terrane boundary crosses the trough and passes between George Bligh and Rockall Banks. Basalts representing uncontaminated mantle-derived material are scarce, but the available data indicate that the Cretaceous basalts of Rosemary Bank and Anton Dohrn have higher 206Pb/204Pb ratios than the younger Palaeocene lavas of Rockall Bank, the Hebrides Shelf and well 163/6-1A, implying significant geographic and/or temporal changes in mantle composition.

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