Marscoite and related rocks of the Western Red Hills complex, Isle of Skye

Abstract

The rock from Skye which Harker called marscoite was recognized by him to be hybrid in origin because of the close association in the rock of basic plagioclase, quartz and orthoclase. The position of marscoite in the sequence of rocks forming the Western Red Hills Tertiary complex has now been defined, and evidence for its parentage and the mechanics of its formation obtained. The Western Red Hills intrusive centre, developed after the Cuillin and before the Broadford centres, as suggested by J. E. Richey, consists of five different, high-level granitic intrusions which were followed by a southern and northern series of late intrusions, including marscoites, ferrodiorites, and various additional granitic rocks. The high-level granitic rocks of Skye, which have so far been loosely termed granophyres, cannot all properly be described as such, and the term epigranite is proposed as a general name for them. The earliest rocks belonging to the southern late intrusions are porphyritic epigranites and felsites, having quartz and potash felspar phenocrysts resembling the xenocrysts of these minerals in the marscoite. Then came marscoite, somewhat chilled against the felsite but also back-veined by it. The marscoite in Harker’s Gully, and in other places on Marsco, passes gradually into ferrodiorite which sometimes contains basic andesine phenocrysts similar to the xenocrysts of the marscoite. The ferrodiorite has a composition suggesting that it was derived by extreme fractional crystallization of basic magma. The xenocrysts of the marscoite are highly characteristic and indicate that marscoite was formed by the mixing of a porphyritic acid magma, like that which produced the Southern Porphyritic Felsite, and a porphyritic basic magma, like that which produced the porphyritic ferrodiorite of Marsco. The chemical compositions of the presumed parent materials and marscoite support this view. Because of the even distribution of the xenocrysts in marscoite, the mixing must have been largely effected by the mechanical stirring together of two liquids, in both of which were suspended crystals. Diffusion within the liquid phase must also have contributed in some degree to the ultimate homogeneity of the liquid part of the mixture. The origin of the marscoite of the northern late intrusions is presumably similar to that of the southern, except that the basic parent is believed to be represented by the porphyritic hawaiite blocks in the northern marscoite. The northern marscoite cuts through, and is chilled against, the Glamaig, Eas Mor, and Maol na Gainmhich epigranites. Inwards from the contacts, the marscoite gives place, in a distance of 30 to 50 yd., to a rock here called glamaigite, which consists of rounded, dark patches, usually an inch or so across, and a less dark matrix, in approximately equal amounts. In both dark and light material there are xenocrysts of andesine, potash felspar and quartz, as in the marscoite. The difference in composition of the darker and lighter parts of the rock is not great, but is such as to suggest that the darker would have had a slightly higher temperature range of crystallization. The glamaigite is believed to have originated from a less well-homogenized mixture of basic and acid porphyritic magmas. From the mixture, the slightly more basic parts solidified first, and then flow movements of the magma resulted in rounding of the early semi-solid clots of hybrid material. The central parts of the composite, northern, late intrusions consist of a rock resembling glamaigite but tending to be more uniform and acid in composition. The greater homogeneity of this rock, distinguished as dioritic glamaigite, may be due to its central position within the intrusions, where slower cooling would allow more time for diffusion. It is suggested that the epigranites of the Hebridean igneous province originated by melting of crustal rocks of broadly granitic composition. The heat to produce the melting is believed to have been derived from basic magma intruded into the earth’s crust, the upward transfer of heat being aided by convection in the magma and bottom accumulation of early formed crystals. At some stage, a residual layer of ferrodiorite or hawaiite liquid, produced by fractionation, may have underlain a granitic liquid produced by melting. Two separate systems of convection currents are envisaged in the two liquids, because of the differing densities. At the junction of the two systems of currents, where they would be flowing in opposite directions, there would be an opportunity for mechanical mixing. Ultimately, a mass of hybrid magma may have developed, annular in form and with a forced circulation, which was the source of the marscoite and related rocks.