Trace Element and Isotopic Variations in Scottish and Irish Dinantian Volcanism: Evidence for an OIB-like Mantle Source

Abstract

The Dinantian volcanic province of southern Scotland, northern England and parts of Ireland is represented by a large volume of transitional to mildly alkaline rocks. Many (although not all) of its basic members display inter-regional variations in incompatible element concentrations, particularly LREE, Zr, Nb, K, and Ba. Sr, Nd, and Pb isotopic data also show some minor inter-regional variations, although they are not as well-defined as those observed in the trace elements. Indeed, much of the variation is intra-regional. Modelling of the basic rocks (≥4% MgO) using Ce/Y and Zr/Nb ratios reveals that fractional crystallization and varying degrees of partial melting alone cannot explain the compositional diversity. Models involving variable-depth melting in the stability fields of both garnet and spinel, and garnet exhaustion through progressive melting are equally untenable. The best model to fit the ratios is one involving variable-degree melting superimposed on slight source region heterogeneity. Investigations of potential magma sources suggest that this heterogeneity is not related to crustal contamination or to incorporation of relict slab material derived from a pre-existing subduction zone (c. 50 Ma earlier) in the same region. Instead, the observed trace element and isotopic characteristics show marked similarities to many modern ocean island basalts (OIB): a feature which suggests that the mantle source region was sub-lithospheric. It is suggested that the variations observed in basic Dinantian volcanic rocks merely mirror on a local scale the variations observed in OIB globally, and that no other source is necessary to explain the chemical diversity of Dinantian magmas. If the speed of northward motion (15° of latitude between the Lower Carboniferous and Lower Permian) and the longevity of Dinantian volcanism (c. 40 Ma) are taken into account, it is hardly surprising that such distinct chemical variations are observed: the magmas may well have sampled an enormous volume of convecting sublithospheric mantle.