Magnetism of igneous rocks from the Tourmakeady and Glensaul inliers, W. Ireland: mode of emplacement and aspects of the

• Main Authors: Ordovician Field Pattern, E. R. Deutsch, K. M. Storetvedt
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Key words; Palaeomagnetism; rock magnetism; Ordovician; W. Ireland; North Pole
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1032.3417; http://gji.oxfordjournals.org/content/92/2/223.full.pdf

Results of palaeomagnetic and rock magnetic studies are reported mainly for quartz porphyry collected at 18 sites from an Ordovician igneous-sedimentary succession at Tourmakeady and Glensaul in'Co. Mayo, Eire. Both magnetite and haematite are shown to be prominent magnetic constituents. The magnetite is usually in the form of large multidomain grains; these are held responsible for a soft remanence component (called the B-magnetization) with D, Z=OOO, +75; ag5=3.70, i.e. practically identical to the direction of the present axial geocentric dipole field in the area. Hence, it is inferred that the B-magnetization, which is dominant at most sites, is of recent origin. Haematite, which has a more patchy occurrence, apparently formed.through high-temperature oxidation of magnetite. Haematite is the main carrier of the palaeomagnetically significant component, the A-magnetization, which gave well-grouped in situ directions (29 samples from 10 sites) with a mean value of D, I = 182", +6", corresponding to a north pole at 31 " N, 169 " E; dp, dm = 2.1", 4.2". Correction for folding along the Glensaul-Tourmakeady arc shows that the A-magnetization is of post-folding age. An earlier suggestion that the rocks represent a Lower Ordovician extrusive suite is discussed and discounted. It is inferred that the quartz porphyries constitute syntectonic intrusions having formed during the Taconic orogeny. These rocks as well as their characteristic magnetization are therefore regarded as late Ordovician in age. It is concluded that a c. SO " relative clockwise shift between the mid-Ordovician and late Ordovician-Silurian geomagnetic field axes, previously demonstrated for Scotland, has now been recognized also in Ireland.