The Southern Mountains Zone, Isle of Rum, Scotland: volcanic and sedimentary processes upon an uplifted and subsided magma chamber roof

The Southern Mountains Zone of the Rum Central Complex lies inside a major ring fault and comprises an intricate association of country-rock outcrops, breccias and rhyodacite. The breccias and rhyodacite were long thought to be products of subterranean explosion and intrusion, respectively. Here, we report new observations that support re-interpretation of these units as mass movement deposits and ignimbrites. The most abundant breccias (Coire Dubh-type) consist mainly of country-rock clasts <1 m in diameter in a sand or silt matrix. Internally bedded and graded, and interlayered with sandstones and lithic tuffs, these breccias are interpreted as debris flow and stream flow deposits. Rhyodacite sheets show gradational or sharp, concordant contacts with Coire Dubh-type breccias, and display graded basal lithic tuffs and graded fiamme swarms. These sheets are interpreted as moderately to densely welded rhyodacite ignimbrites (25–100 m thick). A steep body of fragmented (fiamme-bearing) rhyodacite with intrusive non-fragmented contacts is interpreted as an ignimbrite vent system. The rhyodacite and breccia succession is over 200 m thick and unconformably overlies a structurally uplifted Precambrian basement, within which there is also evidence of later subsidence. Outcrops of potential caldera-collapse ‘megabreccia’ are more structurally consistent than previously thought, and are re-interpreted here as coherent segments of Precambrian country rock (caldera floor). The Southern Mountains Zone breccias and rhyodacites respectively reflect sedimentary and pyroclastic processes acting in response to a complex tectonic interplay of intrusion-related uplift and caldera subsidence.