The Roots of a Magma Chamber, the Central Intrusion, Rum, NW-Scotland

Sammanfattning: The island of Rum in the Inner Hebrides, NW-Scotland, hosts a central volcanic complex that is part of the British-Irish Palaeogene Igneous Province situated in NE- Ireland and NW- Scotland. On Rum, rocks from several stages of Palaeogene magmatic activity have been exposed by millions of years of erosion. Rum is best known amongst geologists because of the famous layered ultrabasic intrusion that covers the SSE part of the island, which is amongst the world’s most studied non-active (fossil) volcanoes. The Long Loch Fault traverses Rum with an N-S direction and has been proposed to represent the feeder zone to the layered ultrabasic intrusion. The area close to the Long Loch Fault has been named the Central Intrusion, and was formed by interaction of the plutonic rocks with the Long Loch Fault. There are two end-member theories for the origin of the Central Intrusion: (i) formation by wholesale subsidence (graben formation) of layered ultrabasic units or (ii) by intrusion of new material in-between the Western and Eastern Layered Intrusions (brecciating and fracturing blocks of material from the layered suite) and so causing pulses of uplift and subsequent collapse. To test how the Long Loch Fault influenced magma emplacement on Rum, field work was conducted including structural mapping, and rock-sampling. The data collected in the field were processed by structural 3D modelling (Move Software suite) and complemented by petrography, FTIR, Electron Microprobe analysis and thermobarometry modelling. The results reveal that several fault splays cut the Central Intrusion, which furthermore provide evidence of a transtensional graben situated above the fault zone and into which the layered units collapsed. This collapse was associated with the intrusion of Ca-rich feldspathic peridotite at zones of weakness in the layered rocks (e.g. bedding planes, unit contacts and fractures), producing smaller fault blocks and acting as a lubricant in-between blocks. FTIR and barometry results show that the intruding feldspathic peridotite magma was water-rich and that the clinopyroxenes in the magma crystallised at approximately 15 km of depth. Consequently, a combination of both theories for the creation of the Central Intrusion appears most reasonable. The combination of data gathered allows to formulate a model in which the tectonic activity of the Long Loch fault repetitively opened and closed the magma conduit, causing pressure build-up in the underlying magma reservoir(s) when the conduit was shut. When the pressure was released, e.g. during fault movement, dense (phyric) and wet ultrabasic magma ascended rapidly and spread out into the shallow crustal magma chamber, thus supplying the growing ultrabasic pluton with pulses of magma from depth.