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The Central Series of the Paleocene mafic to ultramafic Rum complex is the youngest of the three main cumulate series that make up the layered igneous complex of this famed Scottish island. The Central Series lies along the Long Loch Fault and provides insights into the feeder system to the Rum intrusion at an erosion level of about two kilometres below the former land surface. Much of the Central Series consists of a mélange of steep sided bodies of magmatic breccias that stretch along the Long Loch Fault (LLF) in a relatively narrow zone and is composed of blocks and clasts of all sizes derived largely from break‐up of the former conduit walls. Repeated movements of the LLF are thought to have been responsible for opening and closing of the magma conduit, resulting in repeated replenishment events, each of which gave rise to new cumulate formation within the Central Series and the bordering Eastern and Western Layered Series, which crystallized under relatively tranquil conditions. The Central Series probably acted as the feeder zone supplying the neighbouring layered series. The more complete of these is the Eastern Layered Series in which 16 conformable units can be distinguished. Others are presumed either to lie unseen at depth or to have been stripped by erosion. The Central Series, although often neglected because of its relative inaccessibility and complexity, formed from successive magma replenishments alternating with large‐volume side‐wall collapses of previously deposited cumulate material. It could thus be thought of as representing the 'pulsing heart' to the Rum volcano and deserves to be regarded as a site of major volcanological and petrological importance. Here we present a summary of some 60 years of investigation into the cumulate rocks of the Central Series. [ABSTRACT FROM AUTHOR]

Sr and Nd isotope ratios, together with lithophile trace elements, have been measured in a representative set of igneous rocks and Lewisian gneisses from the Isle of Rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of Palaeogene magmatism in the North Atlantic Igneous Province (NAIP). The Rum rhyodacites appear to be the products of large amounts of melting of Lewisian amphibolite gneiss. The Sr and Nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component. Concentrations of the trace element Cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced Cs and Rb loss prior to Palaeogene times, possibly during a Caledonian event. This depletion caused heterogeneity with respect to [sup.87]Sr/[sup.86]Sr in the crustal source of silicic melts. Other igneous rock types on Rum (dacites, early gabbros) are mixtures of crustal melts and and primary mantle melts. Forward Rare Earth Element modelling shows that late stage picritic melts on Rum are close analogues for the parent melts of the Rum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss. These primary mantle melts have close affinities to Mid-Oceanic Ridge Basalts (MORB), whose trace element content varies from slightly depleted to slightly enriched. Crustal anatexis is a common process in the rift-to-drift evolution during continental break-up and the formation of Volcanic Rifted Margins systems. The 'early felsic--later mafic' volcanic rock associations from Rum are compared to similar associations recovered from the now-drowned seaward-dipping wedges on the shelf of SE Greenland and on the Voring Plateau (Norwegian Sea). These three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place. Keywords: North Atlantic Igneous Province, Isle of Rum, Voring Margin, Volcanic Rifted Margins, crustal contamination, crustal anatexis, petrogenesis.

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 Mam 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 Mam 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. Keywords: North Atlantic Igneous Province, Isle of Rum, felsic magmas, intrusion breccia, geochemistry, magma-crust interaction.

The Palaeogene Slieve Gullion Igneous Centre in southern Armagh, Northern Ireland, consists of a layered central intrusive complex surrounded by a prominent and slightly older ring-dyke that intrudes both Lower Palaeozoic sedimentary rocks and the Caledonian Newry Granodiorite pluton (452 Ma). The ring-dyke comprises two major rock types: porphyritic felsite and porphyritic granophyre. We analysed both ring-dyke lithologies, both types of country rock, and a local Palaeogene basalt dyke sample for Sr and Nd isotopes. Trace element and whole rock data for this suite suggest that there are two distinct groups of both felsite and granophyre: one Si-rich and one Si-poor, most likely representing two magmas from a zoned chamber and their mushy chamber wall equivalents (McDonnell et al. 2004). Isotope data show the low-Si rocks to be higher in radiogenic Sr than the high-Si rocks, which is inconsistent with a simple AFC-scenario of increasing sediment assimilation with higher degrees of differentiation. However, using MORB-type basalt as a starting composition, the low-Si ring-dyke rocks can be modelled through AFC with Lower Palaeozoic sedimentary rock as the contaminant. The decreasing 87Sr/86Sr trend from low-Si to high-Si dyke rocks, in turn, represents a second stage of contamination. Selective assimilation of the most fusible portions ofNewry Granodiorite, which is lower in radiogenic Sr than the local sedimentary rocks, appears to be the most plausible solution. The Sr and Nd data are consistent with (a) at least a two-stage contamination history during upper crustal residence and storage, whereby fractionating magmas of basaltic and intermediate composition are contaminated by local sedimentary rocks, giving rise to rhyolite magmas that experience additional shallow contamination by Newry Granodiorite, and (b) a zoned rhyolite magma chamber where high-Si magma is stored in the upper part of the chamber where crystallization and crustal contamination are most extensive. Keywords: ring-dyke, Northern Ireland, crustal contamination, isotopes.

Laterally extensive (∼2 mm thick) Cr-spinel seams in the Rum Layered Suite, NW Scotland, occur at the bases of several of the coupled peridotite–troctolite macro-rhythmic units that form the bulk of the eastern part of the intrusion. Detailed petrography, mineral chemical analyses and quantitative textural measurements of the rocks above and below two of these seams suggest that existing models for seam petrogenesis involving early crystallization and gravitational settling of Cr-spinel from a newly emplaced body of magma need to be reassessed. We argue for assimilation of troctolitic cumulate by a new influx of picrite magma at the crystal mush–magma interface, and subsequent in situ crystallization of the Cr-spinel seams. The bases of seams are characterized by Mg- and Al-rich Cr-spinel, with slightly more Fe-rich crystals toward the tops. These seams crystallized from a superheated hybrid magma generated by the initial assimilation and dissolution of the plagioclase-rich cumulate floor by the picrite. Coeval syn-magmatic deformation of the crystal mush at the unit boundary between peridotite and troctolite caused localized expulsion of Cr-spinel seed crystals several centimetres upward into the peridotite mush, resulting in the formation of overlying supra-seams as well as possibly developing a distinctive texture comprising chains of Cr-spinel around cumulus olivine crystals immediately above unit boundaries. The mineral compositional and textural evidence collected from the Rum Cr-spinel seams also indicates that they exhibit varying degrees of postcumulus chemical and textural equilibration. [ABSTRACT FROM PUBLISHER]