Petrology of the uppermost upper mantle deduced from spinel-lherzolite and harzburgite nodules at Calton Hill,Derbyshire

A suite of protogranular- to porphyroclastic-textured spinel lherzolites and harzburgites from the Lower Carboniferous ankaramite vent at Calton Hill has been investigated for trends of modal variation and mineral composition and for variation in calculated bulk composition. The results indicate that the nodules are accidental xenoliths derived from a source at approximately 45 km depth and at 950 °C, i.e. within the mantle but above the Low Velocity Zone. The lherzolites and harzburgites have a complex petrogenetic history involving initital formation as residues from partial melting of peridotite; it is proposed that the residues were then admixed with veins of pyroxenite, followed by a complex series of metamorphic cycles of mineral reaction and exsolution, deformation, recrystallization and annealing and finally by rupture and incorporation in the ankaramite. During ascent to the surface chrome diopside in some nodules has undergone partial, incongruent melting to form a less sodic pyroxene and a soda-rich basalt melt.