GEOLOGICAL IMPLICATIONS FROM GEOCHEMICAL AND ISOTOPIC STUDIES OF UPPER CRETACEOUS AND LOWER TERTIARY IGNEOUS ROCKS AROUND THE NORTHERN ROCKALL TROUGH

Major, trace, rare-earth element geochemistry and isotopic data have b een obtained for offshore igneous rocks from around the northern Rocka ll Trough, NE Atlantic Ocean. Most comprise basalts derived from the s ub Rockall-Hebridean asthenospheric mantle, but subcontinental lithosp heric mantle generated the phono-tephrites of Rosemary Bank, and melti ng of upper crust was responsible for the peraluminous dacites of well 163/6-1A. Crustal contamination of the lavas around the Rockall Troug h suggests that the Lewisian-Islay (Rockall Bank) basement terrane bou ndary crosses the trough and passes between George Bligh and Rockall B anks. 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 Pb-206/Pb-204 ratios than t he younger Palaeocene lavas of Rockall Bank, the Hebrides Shelf and we ll 163/6-1A, implying significant geographic and/or temporal changes i n mantle composition.