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

The Dinantian volcanic province of southern Scotland, northernEngland and parts of Ireland is represented by a large volumeof transitional to mildly alkaline rocks. Many (although notall) of its basic members display inter-regional variationsin incompatible element concentrations, particularly LREE, Zr,Nb, K, and Ba. Sr, Nd, and Pb isotopic data also show some minorinter-regional variations, although they are not as well-definedas those observed in the trace elements. Indeed, much of thevariation is intra-regional. Modelling of the basic rocks (4%MgO) using Ce/Y and Zr/Nb ratios reveals that fractional crystallizationand varying degrees of partial melting alone cannot explainthe compositional diversity. Models involving variable-depthmelting in the stability fields of both garnet and spinel, andgarnet exhaustion through progressive melting are equally untenable.The best model to fit the ratios is one involving variable-degreemelting superimposed on slight source region heterogeneity. Investigations of potential magma sources suggest that thisheterogeneity is not related to crustal contamination or toincorporation of relict slab material derived from a pre-existingsubduction zone (c. 50 Ma earlier) in the same region. Instead,the observed trace element and isotopic characteristics showmarked 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 Dinantianvolcanic rocks merely mirror on a local scale the variationsobserved in OIB globally, and that no other source is necessaryto explain the chemical diversity of Dinantian magmas. If thespeed of northward motion (15? of latitude between the LowerCarboniferous and Lower Permian) and the longevity of Dinantianvolcanism (c. 40 Ma) are taken into account, it is hardly surprisingthat such distinct chemical variations are observed: the magmasmay well have sampled an enormous volume of convecting sublithosphericmantle.