This study is an investigation into the nature and causes of petrological zonation in calc-alkaline diorite-granite plutons from the Newer granites of the Caledonian orogeny in Scotland. Six plutons were used for the study, namely Garabal Hill-Glen Fyne in the western Highlands, Glen Tilt and Glen Doll in the eastern Highlands, Comrie in the southern Highlands, and Carsphairn and Loch Doon in the Southern Uplands. The petrological zoning is concentric in the Southern Uplands and Comrie and irregular in the other three. The approach has been to acquire data on the petrography, mineral chemistry, and whole rock major and trace element chemistry for representative rock types of the petrological range within each pluton. Variations in mineral compositions are related to equilibrium crystallisation processes and indicate falling crystallisation temperatures with evolving magma composition. Mineral compositions have also been used in chemical models used to explain the observed wide variations in whole rock major oxide compositions. These models are then independently tested using models of trace element behaviour. The principal conclusions are that the main variations within the gabbro-diorite series (including cumulate peridotites and pyroxenites) are best explained by processes of fractional crystallisation from a parental gabbro or diorite magma, but in some cases the more evolved rocks (granites and granodiorites) have a more complex origin including possibly contamination of the parental magma or a distinctive magma source. The assemblage of fractionating minerals in the Garabal Hill, Comrie and Loch Doon plutons is dominated by the relatively anhydrous assemblage of orthopyroxene, clinopyroxene, plagioclase, and biotite whereas in Glen Doll it is a more hydrous assemblage dominated by amphibole. Processes of magma mixing and multi-source pulses are considered appropriate in a few cases. Regional comparisons of the within-pluton compositional variations reveal significant differences. The dominantly calc-alkaline trend shows marked differences in Fe/Mg between plutons as do trace element abundances, reflecting both differences in source region compositions and the influence of fractionating mineral phases. All plutons are I-type and high-K calc-alkaline. Parental magmas for the gabbro-diorite series have features of mantle-derived magmas though the more evolved rock types including granodiorites and granites indicate a significant contribution by crustal anatexis to the magmas.