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

M. S. PETRONIS^*,, B. O'DRISCOLL^,, V. R. TROLL, C. H. EMELEUS^¶, and J. W. GEISSMAN^*

^* Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
Department of Geology, Museum Building, Trinity College, Dublin 2, Ireland
^¶ Department of Earth Sciences, University of Durham, Science Laboratories, South Road, Durham, DHI 3 LE, UK

Author for correspondence: mspetro{at}nmhu.edu; now at Environmental Geology, Natural Resource Management Department, New Mexico Highlands University, P.O. Box 9000, Las Vegas, NM 87701, USA

School of Physical and Geographical Sciences, Keele University, Keele ST5 5BG, UK

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°, ₉₅ = 5.5° that is discordant to an early Paleocene reverse polarity expected field (about 184°, –66°, ₉₅ = 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°, ₉₅ = 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 (K_mean, 29.51 x 10^–3 in SI system), stronger anisotropies (P_j, 1.031) and oblate susceptibility ellipsoids. Group 2 data, from rocks in the west part of the pluton, have lower values of K_mean (15.89 x 10^–3 SI) and P_j (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.

Key Words: palaeomagnetism • anisotropy of magnetic susceptibility • magma emplacement • Isle of Rum • granite

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