No abstract

In 1911 Dr. W. B. Wright briefly described the felspathized quartzite blocks in the hornblendite of Port Easdale, Kiloran Bay, Colonsay, Hebrides. He showed that the exposures provide a conclusive demonstration that the action of hornblendite magma on quartzite xenoliths was to convert them to alkali-felspar and quartz, so that in the final stage ‘one can recognize in numerous angular and rounded patches of felspathic material, without visible xenolithic core, the ghosts of former masses of quartzite’.

Concluded from p. 111.) Vogesite. ! HE vogesites are of two distinct types, one of which will be designated type A and the other type B. Type A.The vogesites of type A show resemblances to the hornblendeporphyrites, towards which they grade by the occasional presence of felspar phenocrysts. They possess a typical panidiomorphic structure. Hornblende, which is similar to that of the porphyrites, builds elongated crystals which may be seven or eight times as long as broad and vary in length from about 2 or 3 mm. to 0-08 mm. or less. The matrix is formed of small lath-like crystals of felspar, amongst which potash-felspar (orthoclase or anorthoclase) predominates, plagioclase also being present. The felspars are stained with haematite, particularly round their margins, so' that they stand out in relief. Quartz and green glass occur interstitially. Magnetite and apatite are again present as accessories, but this time the magnetite appears as small octahedral crystals, whilst the apatite prisms are long and needle-like. The structural characteristic of the rock is the tendency of all the minerals to show form and, with the exception of quartz and magnetite, to be elongated. The larger hornblende crystals have a parallel arrangement due to flow. The serpentinous or chloritic pseudomorphs, seen in the porphyrites, are here even more evident and give definite lozenge-shaped and elongated sections exhibiting the meshwork structure characteristic of olivine. They are again surrounded by reaction rims of hornblende prisms, in which the hornblende is exactly similar to that in the main part of the rock.

Summary The south-western end of the Newry Complex is shown to be composed of hornblende-, biotite-, and porphyritic granodiorite, and the evidence as to the origin of these three varieties is presented. Biotite-granodiorite is found to have developed from biotite-hornfels, sometimes with trondhjemite as an intermediate stage, by the introduction of Na, Ca and Si, with the concomitant removal of Al, Fe, Mg, Na, K, H, Ti, P and Mn. This outgoing material has been fixed in adjacent bands of biotite- and diopside-hornfels, both these rock types becoming basified. As a result of this basification, the complement of granitization, the biotite-hornfels is enriched in biotite and cordierite, whilst the diopside-hornfels is converted first to biotite-hornblende-plagioclase-diabrochite (equivalent to quartz-biotite-diorite) and subsequently to oligoclase porphyroblast biotite-hornblende-plagioclase-diabrochite (equivalent to porphyritic quartz-biotite-diorite). Finally, the granitization of oligoclase porphyroblast diabrochite, by introduction of K and Si, and loss of Al, Fe, Mg, Ca, Na, H, Ti, and P, leads to the development of hornblende-granodiorite. The minimum introductions from an external source necessary for the granitization of the country rocks of the region were Na, Ca and Si, and after granitization was completed migratory material remained which was rich in Al, Fe, Mg and Ca, and included, in addition, Si, Na, K, H, Ti, P and Mn. By analogy with the small-scale complementary changes of granitization and basification, it is suggested that the basic material which migrated from the region of granitization as a whole was fixed (a) in the biotite-enriched contact aureole, and (b) in basic and ultrabasic roof rocks such as those which cap the granodiorites at the north-eastern end of the complex, some of which have already been shown to have been evolved from sediments. The evidence suggests that the processes involved in the evolution of the granodiorites and their complementary basic and ultrabasic types culminate in magma formation.

Synopsis:An interpretation of the Tertiary rocks of the area, which are shown to be denudation relics of a gigantic volcano, is found by following the actualistic principle, that is by interpreting the unknown with reference to the known or actual. “Actualistic”, i.e. with reference to actual causes—commonly used by continental geologists—is chosen in preference to “uniformitarian” since the latter stands in contradistinction to “catastrophic”, and is not, therefore, altogether appropriate to some of the more violent aspects of volcanic activity. Gabbros and granophyres, hitherto thought to be plutonic intrusions emplaced within Caledonian granodiorite, are shown to be highly metamorphosed basaltic and rhyolitic lava-flows, agglomerates and tuffs, together with some gabbro sills. These rocks form a layered series which is almost horizontal at Slieve Gullion, and dips at various angles at Foughill and Carrickcarnan. A Tertiary ring-dyke of granophyre and felsite encircles the layered rocks and outlines a caldera within which the basaltic lavas (including pillow-lavas) of Slieve Gullion accumulated. During the formation of a later caldera these lavas became inset within the underlying Caledonian granodiorite. The acid layers of Slieve Gullion represent rhyolites, incandescent tuff-flows, and agglomerates, the materials of which were derived from the Caledonian granodiorite. The time of eruption of these acid rocks is correlatable with periods of caldera formation.The layered series is dissected by arcuate faults, and contacts previously thought to be intrusive are now demonstrated to be faulted. Transgressive granophyres follow the faults and form intrusion-breccias and net-vein systems with the layered series. Evidence is presented for concluding that these granophyres originated as glassy acid tuffisite—an intrusive pyroclastic rock (Cloos, 1941)—derived from the basement Caledonian granodiorite, and emplaced by rising gas-streams. That the gas sometimes had a temperature high enough to fuse basalt is witnessed by the fact that the basaltic rocks commonly become exceedingly fine grained where they adjoin transgressive granophyre. Such fine-grained edges are cross-cutting to the layers and cannot therefore be interpreted as normal chilled edges. Hybrid rocks, e.g. marscoite, are associated with the transgressive granophyres. They are shown to represent mechanical mixtures of acid tuffisite, carried upwards by rising gas-streams, with fragmented (tuffisitised) and partially fused basic material derived from the walls of the channels. The transgressive granophyres and associated hybrids are related in origin to incandescent tuff-flows, and are regarded as marking the routes by which such flows reached the surface.The metamorphism of the basaltic flows, and the transformation of the acid layers and transgressive acid tuffisites to granophyre, were probably accomplished during the pneumatolytic and hydrothermal phase of volcanic activity. Transformation of the acid rocks to granophyre involved introduction of K and Si, and removal of Na, Al, Fe, Ca, Ti, P and Mn. Similar chemical interchanges transformed basaltic rocks at Foughill and Carrickcarnan to quartz-dolerites. The subtracted materials have been located in small-scale basic fronts margining granophyres; within masses of gabbro that sank within gas-tuff streams from higher levels; and in dispersed form within the basic rocks of Slieve Gullion.