Trace Element Evidence for Mantle Heterogeneity Beneath the Scottish Midland Valley in the Carboniferous and Permian

The alkaline rocks of Carboniferous to Permian age in the Midland Valley province range in composition from hypersthene-normative, transitional basalts to strongly undersaturated basanitic and nephelinitic varieties. They were formed by varying degrees of equilibrium partial melting of a phlogopite peridotite mantle. Ba, Ce, Nb, P, Sr and Zr were strongly partitioned into the liquid during melting; K and Rb were retained by residual phlogopite for small degrees of melting only. The composition of the mantle source is inferred to have been broadly similar to that from which oceanic alkaline basalts are currently being generated. It was, however, heterogeneous as regards distribution of the incompatible trace elements, with up to fourfold variations in elemental abundances and ratios. The mantle beneath the province may be divisible into several areas, of some hundreds of square kilometres each, which retained a characteristic incompatible element chemistry for up to 50 Ma and which imparted a distinctive chemistry to all the basic magmas generated within them.