Petrochemistry of the Early Carboniferous (Dinantian) Lavas of Scotland

Publisher Website
Google Scholar
Abstract
Synopsis: This review is focussed on the early Carboniferous (Dinantian) stage of the complex Carboniferous-Permian volcanic cycle in central and southern Scotland (The “Midland Valley petrographic province”). Stress is particularly laid on aspects of the petrochemistry and petrogenesis not previously recognized or sufficiently discussed in the literature. The scarcity of analytical data for rocks from critical parts of the province means that much of the discussion must be of a preliminary nature; areas where further research is required are indicated. The Dinantian lavas, dominated by a range of mildly undersaturated and hypersthene-normative (transitional) alkaline basalts, have associated nepheline or hypersthene-normative hawaiites, mugearites, benmoreites, quartz-trachytes and rhyolites. Several geographically distinct magmatic lineages are recognized, varying in such parameters as degree of silica undersaturation, Fe/Mg ratios and Na 2 O/K 2 O ratios at comparable values of differentiation index. TiO 2 and P 2 O 5 contents also vary from suite to suite. The proportions of each lava type are variable in different areas, but three volcanic associations are rather arbitrarily distinguished; one showing a spectrum of rocks from ankaramitic basalt to trachytes and rhyolites: a second dominated by plagioclase-phyric basalts and hawaiites: and thirdly, an association consisting largely of olivine-clinopyroxene-phyric basalts, with more differentiated rocks scarce or absent. Though data are very scarce for post-Dinantian lavas, it is suggested that there has been a progressive change in basic magma chemistry with time during the Carboniferous, later alkaline magmas being increasingly silica-undersaturated. This change was accompanied by a change in the nature of the volcanism, which became more explosive, tuffs and agglomerates representing an increasing proportion of the eruptive products. Some aspects of the petrogenesis of the Dinantian basic lavas are examined. A high-pressure (>10 kb) stage of fractionation variably involving clinopyroxene or olivine + clinopyroxene is required to explain the chemical variation in the basalts and hawaiites. More differentiated rocks may have evolved by crystal fractionation at lower pressures (within crustal magma reservoirs?) but insufficient data are available to test this hypothesis.