Magnetostratigraphy of the Tertiary igneous rocks of Arran

Abstract

SynopsisThe Tertiary igneous succession of Arran is poorly k n o w n , partly because of a paucity of informative field relationships, so palaeomagnetic polarities and A r -A r dates have been measured to provide additional constraints.N e w palaeomagnetic results for over 200 sites have been combined with published data. T h e N o r t h e r n Granite has normal polarity (N), while the later Central Ring Complex has reversed polarity (R). All sampled quartz-porphyries are N and all felsites (apart from some intermediate direction (I) sites) are R, the only k n o w n correlations between polarity and petrography in the British Tertiary Igneous Province. The remaining petrographic types may be found with either polarity. All dykes sampled on the N W coast are N , as are 7 0 % of those on the N E coast, but elsewhere R polarity predominates.Radiometric dates show that the activity occurred approximately 61 to 58 Ma ago. It spanned at least the magnetic polarity sequence R-N-R, and although additional polarity intervals cannot be ruled out, a tentative succession consistent with R -N -R and with as few as possible recurrences of petrographic types has been constructed. The mean direction of magnetisation of the sites is not consistent with that of an axial, centred dipole for the present latitude: several explanations are offered.

The North Atlantic Igneous Province reconstructed and its relation to the Plume Generation Zone: the Antrim Lava Group revisited

“…Ages are from the following sources: Mull, Chambers & Pringle (2001); Antrim, this study; Skye, Bell & Williamson (2002); Arran/Muck/Eig/Rhum and Canna, Chambers et al (2005); Faeroe Islands, Storey et al (2007); sLat/sLong, study latitude/longitude; N / n , number of sites/samples; D / I , mean remanence declination/inclination; k and K , Fisher's (1953) precision parameter for the remanence and pole domain; α 95 and A95: half angle of cone of 95 per cent confidence in the remanence and pole domain; plat/pLong: pole latitude/longitude; 60 Ma (°N and °E): Greenland poles rotated into pre‐Atlantic positions with respect to Europe; dp/dm: semi axes of the ovals of 95 per cent confidence in pole positions. References for paleomagnetic data: (1) this study, (2) Løvlie et al (1972), (3) Wilson (1970), (4) Ganerød et al (2008), (5) Rousse et al (in prep), (6) Wilson et al (1972), (7) Wilson et al (1982), (8) Dagley et al (1978), (9) Mussett et al (1987), (10) Hodgson et al (1990), (11) Dagley & Mussett (1986), (12) Dagley & Mussett (1981), (13) Dagley et al (1984), (14) Riisager et al (2002), (15) mean poles from Ganerød et al (2008). Poles marked with an asterisk are the latest contributions by the authors.…”

Time and duration of activity in the British Tertiary Igneous Province

“…Both N and R polarities are found, and the crosscutting relationships within a group of dykes on the S shore show that igneous activity must have spanned at least the sequence R-N-R. Though there is no firm evidence to exclude additional polarity intervals, all the known relationships can be tentatively accommodated within these three polarity intervals (Hodgson 1981). As the northern granite is N, the later and reversely magnetized Central Complex is assigned to the second R interval.…”

Further evidence for a single polarity and a common source for the quartz-porphyry intrusions of the Arran area

“…Thirdly, though it is known that the formation of the Arran Tertiary igneous rocks must have continued through (parts of) three magnetic polarity intervals, R -N -R , there is no compelling evidence that it continued for longer (Hodgson 1981). This reduces the possibility that the quartz-porphyries could have been intruded in more than one normal period.…”

“…Of 435 Arran dykes sampled by Dagley et al (1978), which were probably intruded throughout the period of igneous activity and so are representative of the succession, only 17% are normally magnetised. Of bodies other than the dykes, the only normally magnetised ones known are the Northern Granite, some sites in the trachytic Holy Island Sill (which inexplicably also has sites of reverse and intermediate directions) and, of course, the quartz-porphyries (Hodgson 1981;Mussett et al 1987). The possibility that the normal magnetisation is a later, secondary event can be discounted, for there is no suggestion of overprinting in the demagnetisation behaviour, and both the Judd 4 and the Cleitheadh nan Sgarbh Dykes are cut by reversely magnetised dykes with local overprinting (Mussett et al 1987).…”