ELEMENTAL EVIDENCE FOR AN ENRICHED SMALL-FRACTION-MELT INPUT INTO TERTIARY MULL BASALTS, WESTERN SCOTLAND

Continental basalts enriched in the incompatible trace elements have r ecently been interpreted as mixtures of depleted asthenospheric melt a nd an enriched small-fraction-melt lithospheric component. Hebridean T ertiary basalts are relatively depleted in incompatible trace elements , and it has been suggested that this is because the enriched lithosph eric component had already been extracted beneath that region, during the Permo-Carboniferous. It was therefore not widely available to cont aminate Tertiary magmas. A flow-by-flow geochemical study of the Mull lava succession has nevertheless revealed the presence of lava flows r elatively enriched in the incompatible trace elements, at the base of the succession, in some parts of the island. Fractional crystallizatio n, contamination by Archean and Moinian crust, variation in the degree of mantle melting and an asthenospheric, ocean island basalt-like sou rce, have all been ruled out as possible mechanisms of enrichment in f avour of 5-10% contamination by an enriched, fusible, small-fraction-m elt from the lithospheric mantle. The overall lack of basalts enriched in trace elements in the British Tertiary Igneous Province, argues st rongly against the widespread presence of an enriched Tertiary lithosp heric mantle below the Hebrides. The origin of the a sodic (rather tha n potassic) contaminant within the Hebridean lithosphere is also discu ssed.