The extent to which interaction occurred between the proto-Iceland mantle plume and the continental lithospheric mantle (CLM) during establishment of the Palaeogene British and Irish Igneous Province (BIPIP) has wide-ranging and important implications, not least for understanding the nature and differentiation of the parental melts, models of contamination of primary magmas as well as base and precious metal mineralization. This study focuses on a rare Palaeogene-aged xenolithic dyke from Inver, NW Ireland. Geothermobarometric calculations indicate that spinel lherzolites from the Inver dyke represent relatively fertile, cold, deep CLM. The Al-in-olivine thermometer yields 865–974°C for two xenoliths studied in detail and equilibration pressures (using the Ca-in-olivine-clinopyroxene barometer) of 12–15 kbar. Oxygen fugacity calculations indicate relatively reducing conditions (Δlog f O2[QFM]) of −3.2 to −2.3, a characteristic not inconsistent with deeply sourced CLM materials. Thermobarometry on the dyke host rock suggests that its parental melt equilibrated with the mantle at 1326°C and 18.6 kbar, constraining the depth to the lithosphere-asthenosphere boundary at ∼43 Ma to ∼59 km. Taken together, these observations attest to the localized survival of relatively unmodified CLM following the main pulse of BIPIP magmatism at 61–58 Ma. The new constraints are permissive of magmatic mineral deposit formation or enhancement by exchange between locally metasomatized CLM and rising asthenospheric melts, but such processes may be localized, i.e. beneath individual igneous centres of activity.

The geochemistry of late Caledonian minettes from across the orogenic belt is compared in order to constrain the composition of the Caledonian sub-continental lithospheric mantle (SCLM). All the minettes are similar petrographically and chemically and several samples have characteristics typical of near primary mantle melts. Samples from the Northern Highlands and the Caledonian foreland show enrichment in many trace elements (notably LILE and LREE) relative to those from the Grampians, the Southern Uplands and northern England, coupled with distinct Nd and Sr isotope characteristics. Processes such as fractional crystallization, crustal assimilation, and partial melting played a negligible role in creating the differences between the two groups which reflect long-term, time-integrated differences in the compositions of their SCLM sources. The Great Glen Fault appears to represent the boundary between these two lithospheric mantle domains. Other currently exposed Caledonian tectonic dislocations cannot be correlated directly with compositional changes within the SCLM. The chemical provinciality displayed by the minettes shows some resemblance to that within other late Caledonian igneous suites, including the newer granites, suggesting that the minettes may represent the lithospheric mantle contributions to these rocks.