Petrology and geochemistry of a cumulate xenolith suite from Bute: evidence for late Palaeozoic crustal underplating beneath SW Scotland

Xenoliths from Hawk's Nib (Bute, SW Scotland), entrained in alkali basalt of late Carboniferous-early Permian age, generally possess unrecrystallized igneous cumulate textures. Most are inferred to derive from lower crustal (20-30 km) depths, with a small minority from the underlying restitic mantle. Compositions range from ultramafic cumulates (dunite, wehrlite, websterite, clinopyroxenite) to gabbros and anorthosites. The suite is unique in being dominated by poikilitic wehrlites and olivine clinopyroxenites. Major element variation in the cumulates is controlled by modal mineral variations. The ultramafic cumulates all have high REE contents and light REE (LREE)-enriched patterns, with websterites being the least enriched. Gabbroic xenoliths have uniform patterns in which both LREE and heavy REE are depleted relative to the middle REE, with small positive Eu anomalies. Present-day Sr-Nd isotopic data for the suite are the most depleted among Scottish xenoliths. The xenoliths are inferred to be products of young replenishments of the lower crust by basaltic magmas during continental underplating. Orthopyroxene-bearing lithologies crystallized from tholeiitic magmas whereas alkali basalts formed the clinopyroxene-rich samples; both parental magma-types were derived from similar mantle sources.