Problems of Carboniferous-Permian volcanicity in Scotland
Authors: Macgregor Archibald Gordon 
Published: October 2009

In this paper a first attenpt is made to account for the eruptive sequence of Scottish Carboniferoius-Permian igneous rocks by means of a synthesis involving the mechanism of eruption as well as the broader aspects of differentiation.

The time-sequence of extrusion and intrusion is considered in detail and summarized graphically in tabular form. The rocks concerned are regarded as including camptonite-monchiquite and quartz-dolerite dyke suites widely distributed in the Highlands and Islands.

The main facts to be accounted for are : (1) the wide distribution of felsic differentiates of alkaline basaltic magma, in the form of lavas and intrusions, in early Calciferous Sandstone times (Clyde and Forth areas), as contrasted with the absence of felsic lavas and intrusions from the products of late Calciferous Sandstone, Carboniferous Limestone, Millstone Grit and Permian volcanicity; (2) the availability, for extrusion, of alkaline basaltic magma long before (Calciferous Sandstone to Millstone Grit times) and shortly after (Permian times) the period of intrusion of calc-alkaline quartz-dolerite; (3) the common occurrence of alkaline basaltic magma as lava flows in Carboniferous and in Permian times, as contrated with the absence of extrusive equivalents of the Permo-Carboniferous quartz-dolerite dykes and sills; (4) the occurrence of teschenite almost exclusively as sills, and or kylitic rocks mainly as sills (but in a few instances as isolated dykes or elongated plugs), as contrasted with the prevalence of quartz-dolerite in the form of long wide dykes, some of which represent the feeding channels of adjacent sills; (5) the frequent occurrence in monchiquitic intrusions (dykes, etc.) and in Permian neck-agglomerates, of xenoliths or large xenocrysts of peridotite, pyroxenite, alkali-felspar, biotite, hornblende and augite, and of local country rocks; (6) the availability, for dyke-formation, of alkaline basaltic magma (in the form of basanite, camptonite, monchiquite etc.) over the greater part of Scotland in late Carboniferous or early Permian times, very shortly after the intrusion of calc-alkaline basaltic magma (in the form of quartz-dolerite and tholeiite dykes) throughout much of the same area.

In the attempt to account for these facts, crustal layering of one of the types suggested by W. Q. Kennedy and E. M. Anderson is assumed to exist. Primary magma is envisaged as rising, along vertical crustal fissures, from horizontal sheet-like magma reservoirs of wide extent, situated at depths between 29 and 35 kilometres. The crustal fusion that formed these reservoirs is assumed to have taken place at different levels at different periods, but never simultaneously at different levels. Differentiation is assigned to a high level in the crust, at the relatively cool tops of magma-filled fissures; it is assumed to have taken place during pauses in the rise of primary basaltic magmas derived from parts of a graded Lower Intermediate (Alkaline olivine-basalt) Layer and from the base of the Upper Intermediate (Tholeiitic) Layer. Differentiation of the calc-alkaline tholeiitic magma was negligible.

Violently explosive eruptions of near-surface origin are regarded as controlled by the extent of felsic-mafic layering due to advanced crystallization-differentiation. Evidence is given indicating that Permian volcanic vents originated on monchiquite dykes, owing to a release of explosive energy that may have occurred at great depths.

Local evidence is cited against R. A. Daly's suggestion that Scottish Permian necks originated from satellitic sill-intrusions.

S. I. Tomkeieff, in his petro-chemical synthesis of the Carboniferous and Permian igneous rocks of the Midland Valley of Scotland, appears to postulate one or more layered (differentiated) magma reservoirs with teschenitic magma at the top and quartz-doleritic magma at the bottom. It appears very difficult to imagine an eruptive mechanism that will reconcile such a conception with the temporal and (extended) geographical distribution of magmas and rocks summarized above.