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

Published online by Cambridge University Press:  11 December 2008

M. S. PETRONIS ,
B. O'DRISCOLL ,
V. R. TROLL ,
C. H. EMELEUS  and
J. W. GEISSMAN
M. S. PETRONIS*
Affiliation:
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
B. O'DRISCOLL*
Affiliation:
Department of Geology, Museum Building, Trinity College, Dublin 2, Ireland
V. R. TROLL
Affiliation:
Department of Geology, Museum Building, Trinity College, Dublin 2, Ireland
C. H. EMELEUS
Affiliation:
Department of Earth Sciences, University of Durham, Science Laboratories, South Road, Durham, DHI 3 LE, UK
J. W. GEISSMAN
Affiliation:
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
*
†Author for correspondence: [email protected]; 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
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Abstract

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.

Keywords

palaeomagnetismanisotropy of magnetic susceptibilitymagma emplacementIsle of Rumgranite
Type
Original Article
Information
Geological Magazine , Volume 146 , Issue 3 , May 2009 , pp. 419 - 436
Copyright
Copyright © Cambridge University Press 2008

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