Magnetic polarity stratigraphy of the Tertiary igneous rocks of Skye, Scotland

Abstract

S U M M A R Y Palaeomagnetic results are given for 115 sites, mainly from the central igneous intrusive complex, and also radiometric dates by the 40Ar-39Ar step-heating method. These are combined with published dates and palaeomagnetic results for the lavas and regional dyke swarms with the aim of establishing the magnetic polarity stratigiaphy.Igneous activity spanned at least the approximate interval 61-54 Ma, and a polarity sequence of at last R-N-R-N-R, but could span seven polarity intervals, R-N-R-N-R-N-R, depending on the age of the Loch Ainort Epigranite. However, these polarities cannot be identified with specific chrons of either the Harland et al. (1982) or Berggren et al. (1985) polarity time-scales.The Skye igneous activity, therefore, spans more polarity intervals than in Mull, Ardnamurchan or Rhum and this is consistent with the radiometric ages.Interpretations of the results for the Coke Uaigneich and related granites and for the Loch Ainort Epigranite present problems, and partial solutions are offered.

Paleomagnetic dating of the smectite‐to‐illite conversion: Testing the hypothesis in Jurassic sedimentary rocks, Skye, Scotland

“…In south Skye, where rocks are illite‐rich, a CRM in magnetite is present, but it is absent in north Skye where smectite is still abundant. The time of remagnetization in the early Tertiary coincides with the timing of the igneous activity in the early Tertiary [ Dagley et al , 1990; Wilson et al , 1972], the same time as the inferred illitization of smectite in south Skye [ Andrews , 1987]. The similarity in directions suggests that the clay diagenesis has been dated.…”

“…(a) A stereonet projection of specimen directions (in situ) for a representative site in south Skye; site EL11, (b) two projections of site mean directions along with the means for the (right) intermediate‐ and (left) high‐temperature components, and (c) a projection with the mean (in situ) specimen directions for the high‐temperature (diamonds) and intermediate‐temperature (triangles) components from this study and the mean directions (squares) from Dagley et al [1990]. Open symbols represent negative inclinations, and solid symbols represent positive inclinations.…”

“…The Cuillin is noted for its serrated peaks composed of gabbro and the Red Hills are smoothly contoured mountains composed of granite and granophyre [ Richey , 1961]. These volcanic mountains are between 54 and 61 Myr old and the igneous activity spanned at least five polarity intervals [ Dagley et al , 1990]. The igneous rocks are interpreted to contain a primary magnetization [ Dagley et al , 1990].…”

“…A presence‐absence test was based on comparisons of the paleomagnetic and rock magnetic characteristics of the sedimentary rock between north Skye and south Skye. A timing test was based on comparison of the directions from the altered rock in south Skye with the directions in the Tertiary intrusions [e.g., Dagley et al , 1990]. Petrographic and scanning electron microscopy (SEM) analyses were also used to characterize the rocks and identify alterations of minerals.…”

Structure of a hypovolcanic acid complex inferred from magnetic susceptibility anisotropy measurements: the Western Red Hills granites (Skye, Scotland, Thulean Igneous Province)

“…All these granite units are reversely magnetized, with the exception of the Loch Ainort epigranite (unit V; Fig. 2a; Dagley et al 1990). …”