The Rum Igneous Centre, Scotland

Abstract

The publication of the British Geological Survey memoir on Rum and the Small Isles in 1997 was followed by a period of intense petrological and mineralogical research, leading to some 40 papers, books and other publications. The research progress since then is reviewed here and integrated with the information previously available to provide an overview of the current status of understanding of the centre. New data on the acidic and mixed acid/basic magmas of the early Rum caldera demonstrate that frequent mafic replenishments were the main driver for magmatic activity at Rum right from its initial stages. The caldera is bound by the Main Ring Fault, a structure which probably also exercised an influence on the emplacement of the subsequent basic and ultrabasic intrusions. The later emplacement of gabbros and ultrabasic rocks caused only limited thermal metamorphism of the surrounding Torridonian sandstones, contrasting markedly with the crustal isotope signatures of the early intracaldera ignimbrite magmas and the intense alteration of uplifted masses of Lewisian gneiss within the ring fault. Rare picritic dykes provide an indication of the possible parent magma for the mafic and ultrabasic rocks, but these, as with most other magmatic rocks on Rum, have undergone varying degrees of crustal contamination, involving both Lewisian granulite and amphibolite-type crust but, notably, no Moine metasedimentary compositions as is the case at the nearby Ardnamurchan centre. Detailed textural studies on the gabbroic and ultrabasic rocks allow a distinction between intrusive peridotites and peridotite that forms part of the classic layered cumulate units of Rum and, furthermore, this work and that on the chromite seams and veins in these rocks shows that movement of trapped magma and magma derived from later intrusions, may produce textures regarded previously as of primary cumulate origin. Sulfides in the chromitite seams and ultrabasic rocks, in turn, show possible influences from assimilated Mesozoic sediments. Igneous activity on Rum was short-lived, possibly only between 0.5 and 1 m.y. in duration and commenced at~60.5 Ma. The Rum Central Complex was extinct by the time the main activity at the nearby Skye Central Complex commenced (~59 Ma). From recent apatite fission-track studies it seems probable that Rum, in common with other Palaeogene centres, underwent a brief, but significantly later heating event (~45 Ma).

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

“…Despite the lack of layer continuity typical of sills in the ELI and WLI, intrusive peridotite is likely a major component of the CI (Volker and Upton 1990), contrary to the classic model (e.g. Emeleus and Troll 2014).…”

“…forming bottom to top) within a large body of crystal-poor magma (e.g. Dunham and Wadsworth 1978;Faithfull 1985;Tait 1985;Emeleus et al 1996;Holness 2005Holness , 2007Emeleus and Troll 2014). This paradigm was first challenged by Bédard et al (1988) who established, based on field evidence, that the Unit 9 and 10 peridotites represented sill-like intrusions, later corroborated for Unit 9 by textural and geochemical evidence in the overlying allivalite (Holness 2005;Holness et al 2007;Leuthold et al 2014).…”

Persistent multitiered magma plumbing beneath Katla volcano, Iceland

“…At Katla this phenomenon is underlined by actively deforming silicic cryptodomes in the periphery of the caldera. This arrangement is consistent with the recent encounter of a rhyolite magma pocket as shallow as 2 km beneath the Krafla caldera during geothermal exploration drilling [ Zierenberg et al ., ], and with the architecture of fossil magma chambers in eroded caldera volcanoes elsewhere in the North Atlantic Igneous Province [e.g., Burchardt et al ., ; Emeleus and Troll , ]. Many shallow sub‐caldera storage systems on Iceland likely contain a basaltic heart and peripheral rhyolitic pockets, or cryptodome complexes, which could explain the dominantly basaltic Holocene eruption record at Katla as well as the occasional SILK events [cf.…”

Hydrogen analysis in nominally anhydrous minerals by transmission Raman spectroscopy

“…The included rock samples are El Hierro island ankaramite lava bombs from Tanganasoga volcano (Carracedo et al 2001;Longpré et al 2009;Weis et al 2016). To get a comparison for different crystal chemistries, a clinopyroxene crystal from a gabbro from the well-known Rum igneous intrusion was added (e.g., Emeleus and Troll 2014). All studied clinopyroxene crystals are diopsides (cf.…”