Petrography and mineral chemistry as indicators of variations of crystallization conditions in the Loch Lomond and Appin appinite suites, western Scotland

Petrography together with rock and mineral chemistry of the appinite suite and associated lamprophyres in the Loch Lomond district of western Scotland assist in elucidating the nature of the parent magma and the evolutionary history of the rock types, and provide a basis for comparison with the appinite suite in its type locality (Appin district), also in the Scottish Caledonides. In both suites there are intrusions that are pipe-like and emplaced into subvolcanic pipes of explosion-breccia formed when structural traps were breached following the build up of gas pressure during magmatic crystallization. The Loch Lomond suite crystallized from a magma that was calc-alkaline, but with less alkalinity than the magma from which the Appin suite developed. The Ca contents of the clinopyroxenes in it are lower, Ca-rich plagioclase appeared earlier in its crystallization history and the gas pressure (PH2O <l–4 kb) required to explosively breach the different structural traps was generally lower than that needed to breach many of the traps in the Appin district. However, similarities between the suites are much greater than differences and, in both, there is a comparable wide variety of rock types from ultramafic rocks to appinites with their abundant idiomorphic amphibole, kentallenite, diorites and minor granodiorite, but with pyroxene-mica diorite being the most abundant type in the Loch Lomond suite.