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

The Rudh' a Chromain sill is a composite minor intrusion emplaced into Jura ssic 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 developm ent 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 di stinct magma end members were involved in the Formation of the sill: a thol eiitic basaltic andesite magma generated by contamination and Fractional cr ystallization of regionally available olivine tholeiite basaltic magma, and a rhyolitic magma produced predominantly through crustal melting of baseme nt metasediments. Intermediate compositions at the gradational boundaries b etween the basic margins and silicic interior of the sill formed by high te mperature diffusive hybridization within a compositionally-zoned magma rese rvoir prior to sheet emplacement. The basic portions of the sheet are: repl ete with a large variety of crustal xenoliths, as well as numerous gabbroic and noritic cumulate fragments. The Rudh' a' Chromain sill therefore prese rves evidence for the complex interplay between a number of magmatic proces ses which, when combined with data from the remainder of the suite, suggest s that the magmatic plumbing system which fed the sill complex was not simp ly one large, long-lived magma chamber, but rather a plexus of variably con nected sheet-like magma reservoirs.