Composite minor intrusions as windows into subvolcanic magma reservoir processes: mineralogical and geochemical evidence for complex magmatic plumbing systems in the British Tertiary Igneous Province

Abstract

The Rudh’ a Chromain sill is a composite minor intrusion emplaced into Jurassic sandstones on the south coast of the Ross of Mull, NW Scotland, and is part of a suite of basic–silicic sheets associated with the early development of the 58–56 Ma Mull Central Igneous Complex. New whole-rock major and trace element data combined with Sr–Nd–Pb isotope data indicate that two distinct magma end members were involved in the formation of the sill: a tholeiitic basaltic andesite magma generated by contamination and fractional crystallization of regionally available olivine tholeiite basaltic magma, and a rhyolitic magma produced predominantly through crustal melting of basement metasediments. Intermediate compositions at the gradational boundaries between the basic margins and silicic interior of the sill formed by high temperature diffusive hybridization within a compositionally-zoned magma reservoir prior to sheet emplacement. The basic portions of the sheet are replete with a large variety of crustal xenoliths, as well as numerous gabbroic and noritic cumulate fragments. The Rudh’ a’ Chromain sill therefore preserves evidence for the complex interplay between a number of magmatic processes which, when combined with data from the remainder of the suite, suggests that the magmatic plumbing system which fed the sill complex was not simply one large, long-lived magma chamber, but rather a plexus of variably connected sheet-like magma reservoirs.