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Early mafic magmatism and crustal anatexis on the Isle of Rum: evidence from the Am Màm intrusion breccia

GRAEME R. NICOLL^*,, MARIAN B. HOLNESS, VALENTIN R. TROLL^*,, COLIN H. DONALDSON^¶, EOGHAN P. HOLOHAN^*,#, C. HENRY EMELEUS^**, and DAVID CHEW^*

^* Department of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland
Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
Department of Earth Sciences, Uppsala Universitet, Villavägen 16, 752 36 Uppsala, Sweden
^¶ School of Geosciences, University of St. Andrews, KY16 9AL, UK
^# School of Geological Sciences, University College Dublin, Dublin 4, Ireland
^** Department of Earth Sciences, University of Durham, Durham DHI 3LE, UK

Author for correspondence: g.nicoll{at}ed.ac.uk; current address: School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3SW, UK.

The Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern Mountains Zone), along with the later central ultrabasic–basic layered intrusions. These marginal complexes represent the remnants of near-surface to eruptive felsic magmatism associated with caldera collapse, examples of which are rare in the North Atlantic Igneous Province. Rock units include intra-caldera collapse breccias, rhyolitic ignimbrite deposits and shallow-level felsic intrusions, as well the enigmatic ‘Am Màm intrusion breccia’. The latter comprises a dacitic matrix enclosing lobate basaltic inclusions (~1–15 cm) and a variety of clasts, ranging from millimetres to tens of metres in diameter. These clasts comprise Lewisian gneiss, Torridonian sandstone and coarse gabbro. Detailed re-mapping of the Am Màm intrusion breccia has shown its timing of emplacement as syn-caldera, rather than pre-caldera as previously thought. Textural analysis of entrained clasts and adjacent, uplifted country rocks has revealed their thermal metamorphism by early mafic intrusions at greater depth than their present structural position. These findings provide a window into the evolution of the early mafic magmas responsible for driving felsic magmatism on Rum. Our data help constrain some of the physical parameters of this early magma–crust interaction and place it within the geochemical evolution of the Rum Centre.

Key Words: North Atlantic Igneous Province • Isle of Rum • felsic magmas • intrusion breccia • geochemistry • magma–crust interaction

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