Palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the Western Granite, Isle of Rum, NW Scotland

• Published in: Geological magazine 2009-05, Vol.146 (3), p.419-436
• Main Authors: PETRONIS, M. S., O'DRISCOLL, B., TROLL, V. R., EMELEUS, C. H., GEISSMAN, J. W.
• Format: Article
• Language: English
• Subjects: Anisotropy; anisotropy of magnetic susceptibility; Berggrundsgeologi och petrologi; Cenozoic; deformation; demagnetization; Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics; Earth sciences; emplacement; Endogen geovetenskap; Endogenous earth sciences; Europe; fabric; geochronology; Geophysics; Geovetenskap; Geovetenskap med inriktning mot mineralogi, petrologi och tektonik; Granite; granites; Great Britain; Hebrides; Highland region Scotland; igneous and metamorphic rocks; igneous rocks; Inner Hebrides; intrusions; Inverness-shire Scotland; Isle of Rum; isothermal remanent magnetization; large igneous provinces; layered intrusions; lineation; lithostratigraphy; Magma; magma emplacement; magmatism; magnetic properties; magnetic susceptibility; Magnetism; magnetization; NATURAL SCIENCES; NATURVETENSKAP; North Atlantic igneous province; palaeomagnetism; Paleobiology; Paleocene; Paleogene; Paleomagnetism; petrology; plutonic rocks; relative age; remanent magnetization; Rhum; Rocks; Rum Complex; Scotland; solid Earth (tectonophysics); Solid earth geology and petrology; stratigraphic units; Tertiary; textures; United Kingdom; volcanic rocks; Western Europe; Western Granite
• ISSN: 0016-7568; 1469-5081; 1469-5081
• DOI: 10.1017/S0016756808005827

The Western Granite is the largest of several granitic bodies around the margin of the Rum Central Igneous Complex. We report palaeomagnetic and anisotropy of magnetic susceptibility (AMS) data that bear on the emplacement and deformation of the Western Granite. The collection includes samples from 27 sites throughout the Western Granite, five sites in adjacent feldspathic peridotite, and two sites in intermediate to mafic hybrid contact aureole rocks. Palaeomagnetic data from 22 of the 27 sites in the granite provide an in situ group mean D = 213.2°, I = −69.5°, α95 = 5.5° that is discordant to an early Paleocene reverse polarity expected field (about 184°, −66°, α95 = 4.3°). The discrepancy is eliminated by removing an inferred 15° of northwest-side-down tilting about a best fit horizontal tilt axis trending 040°. Data from the younger peridotite and hybrid rocks of the Rum Layered Suite provide an in situ group mean of D = 182.6°, I = −64.8°, α95 = 4.0°, which is statistically indistinguishable from an early Paleocene expected field, and imply no post-emplacement tilting of these rocks since remanence acquisition. The inferred tilt recorded in the Western Granite, which did not affect the younger Layered Suite, suggests that emplacement of the ultrabasic rocks resulted in roof uplift and associated tilt of the Western Granite to make space for mafic magma emplacement. Magnetic fabric magnitude and susceptibility parameters yield two subtle groupings in the Western Granite AMS data set. Group 1 data, defined by rocks from exposures to the east and south, have comparatively high bulk susceptibilities (Kmean, 29.51 × 10−3 in SI system), stronger anisotropies (Pj, 1.031) and oblate susceptibility ellipsoids. Group 2 data, from rocks in the west part of the pluton, have lower values of Kmean (15.89 × 10−3 SI) and Pj (1.014), and triaxial susceptibility ellipsoids. Magnetic lineations argue for emplacement of the granite as a tabular sheet from the south–southeast toward the north and west. Moderate to steeply outward-dipping magnetic foliations, together with deflection of the country rock bedding in the north, are consistent with doming accompanying magma emplacement.