Hybridization and the petrogenesis of composite intrusions: the dyke at An Cumhann, Isle of Arran, Scotland

Abstract

The composite dyke at An Cumhann, Arran consists of a central unit of quartz-feldspar porphyry flanked by narrow marginal dolerites containing xenocrysts of quartz, plagioclase and alkali feldspar identical to the phenocrysts in the porphyry. The chemistry of the marginal dolerites indicates that they formed by the crystallization of a hybrid magma produced at depth by the incorporation of the porphyritic acid magma in a basic liquid. During the intrusion of the hybrid magma, flow caused a differential distribution of the xenocrysts across the initial basic dyke. Subsequent intrusion of the quartz-feldspar porphyry magma along the still unconsolidated centre of the basic dyke produced the wide central unit. A similar origin is proposed for other composite intrusions of the same type in the region. Whilst it is unlikely that the coexistence of the highly contrasting magmas necessary for the formation of these intrusions is entirely coincidental there is little likelihood that the two liquids were related as members of the same fractionation series.

Palaeomagnetism and age of the quartz-porphyry intrusions, Isle of Arran

“…4). Kanaris-Sotiriou and Gibbs (1985) agree with these suggestions. The dyke is bordered on its eastern side by a thick (c. 8 m) felsite dyke and is cut roughly perpendicularly by a basic dyke (sampled at site 106) which, though it does not penetrate the felsite dyke, post-dates it, for the end of the cross-cutting dyke is chilled against the felsite.…”

“…In those cases where a.f. demagnetisation of one specimen could be compared with thermal demagnetisation of a second specimen from the same core, the thermal cleaning generally gave less well defined directions, though agreeing with the directions obtained using alternating fields, (1893), Tomkieiff (1969) and Kanaris-Sotiriou and Gibbs (1985), with quartz-porphyry shown dotted and basic margins and dykes in black. and contributed no new information.…”

“…However, as the thick felsite dyke (sites 108, 41 OC) is also R and was probably intruded after the composite dyke (Tyrrell 1928, p. 200;Kanaris-Sotiriou and Gibbs 1985) it probably overprinted the adjacent part of the composite dyke. Support for this can be found in the traverse from the felsite, through the basic margin, and into the quartz-porphyry (410): the felsite and margin are clearly R while the quartz-porphyry has an intermediate direction at site 410A-4, 0-56 m from the felsite, and normal directions at sites 410A-3 and -2, 0-6 and 0-7 m respectively, from the felsite.…”

Magma Ascent along a Major Terrane Boundary: Crustal Contamination and Magma Mixing at the Drumadoon Intrusive Complex, Isle of Arran, Scotland

“…Mull and Slieve GullionçEmeleus, 1962;Sparks, 1988;McDonnell et al, 2004) and thick sill complexes (e.g. Arran and Buteç Smellie, 1914;Harrison, 1925;Kanaris-Sotiriou & Gibb, 1985). At these central complexes magma suites are often bimodal in composition (basalt^rhyolite), whereas intermediate compositions are both spatially and volumetrically rare, a phenomenon known as the 'Bunsen^Daly gap' (Bunsen, 1851;Daly, 1925).…”

Gazetteer of Arran Pitchstone Sources: presentation of exposed pitchstone dykes and sills across the Isle of Arran, and discussion of the archaeological relevance of these outcrops

“…Highly porphyritic, yellowish-brown. Kanaris-Sotiriou & Gibb 1985. Illus 40 Diagrammatic plan of Judd's No. I Dyke: a = tholeiite; b = banded and spherulitic felsite (andesite); c = pitchstone (andesite); s = sandstone (Judd 1893, fig 2).…”