Summary The post-Cambrian sills and dykes described occur mainly in the Assynt district of the North-West Highlands of Scotland. The principal types are grorudites, Canisp porphyry (biotite, albite-oligoclase, aegirine-augite porphyrite), hornblende-porphyrites, nordmarkitic rocks, vogesites and ledmorites and related rocks. They are emplaced in Lewisian Gneiss, Torridon Sandstone and Cambrian sediments, and some also occur in the Moine schists. The intrusions are affected by the Caledonian thrust movements. The tectonic distribution of the grorudites leads to the recognition of the Glencoul and Ben More thrust-masses as parts of a single tectonic unit. South of Conival the Benmore thrust-plane is of greater importance than to the north, and is renamed the Assynt thrust-plane. The sills in the Moine Series are metamorphosed and are thought to have been emplaced after the inception, but before the close, of the Caledonian thrust movements; some part at least of the Mo^ne metamorphism is thus of post-Cambrian age. Assimilation of limestone has probably no relation to the production of lime-magnesia pyroxene in the vogesites, nor to the production of highly alkaline rock types. A mechanism of differentiation of the post-Cambrian suite is suggested. Both major and minor intrusions have been emplaced by injection; in places the thickness of the sills amounted to several hundred feet, and had considerable effect in up-doming the strata over which the Moine schists were thrust.
The Tertiary dolerite plug at Carneal cuts basalt lavas and incorporates blocks of chalk and flint from underlying Cretaceous rocks. Assimilation by the dolerite of the pure limestone and flint took place under the very rare highest-temperature, low-pressure conditions. Only about twenty-five examples of the resulting metamorphosed and metasomatized rocks are known in the world, few fully described. The rock suite enables the conditions and mechanisms of assimilation to be deduced. The pressure, about 200 x 10 5 Pa (200 bar), and the temperature, estimated as 1050-1100 °C, produced an exomorphic suite of larnite, spinel, merwinite, spurrite, scawtite and related assemblages, with wollastonite, quartz, plagioclase, hydrogrossular, xonotlite and related minerals representing flint. Complementary desilication of the igneous rocks gives the endomorphic suite of pyroxene-rich dolerite, pyroxenite, titanaugitemelilite-rock and aegirine- and nepheline-bearing types. Chemical analyses o titanaugite, sahlite, melilite, wollastonite and the main rock types are provided and optical and other properties of the minerals. Two related mechanisms of limestone assimilation occurred. Most of the rocks resulted from the incorporation of the chalk in the olivine-dolerite magma, paradoxically, the addition of 18-26 % CaO to the dolerite magma so lowers the silica ratio that 17-18% additional S i0 2 is required to produce the endomorphic hybrids, with the complementary exomorphic suite. The second mechanism, a metasomatic replacement, preserves existing mineral (and fossil) textures. Mineral textures and the preservation of a cyclostome bryozoan now composed of wollastonite (by silicification of calcite) show the metasomatism to have been a tranquil process despite the high temperature. To produce the vein assemblage of merwinite, hydrogrossular and melilite, this mechanism required 63 % CaO and 23 % H 2 0 , an addition which is only slightly more hydrous than calcium hydroxide. These mechanisms are evidence for the production of peralkaline rocks by limestone assimilation but only on a very small scale. The retrograde phase of metamorphism produced minerals in order of approximately increasing water content, including xonotlite, bicchulite (a new mineral), thomsonite, tobermorite, tacharanite and plombierite.
ABSTRACT. Bredigite is a constituent of the very hightemperature, low-pressure, exomorphic suite of Carneal, Co. Antrim. Although this mineral is very rare in nature, it is an important constituent of some slags and cement clinkers but there has been much controversy about its nature, most of the evidence having come from artificial materials. Chemical analysis of the Carneal mineral shows it to be remarkably similar to that from the type locality, Scawt Hill (also analysed here), and that it is an individual mineral species of generalized ionic composition (Ca,Na)14(Mg,Fe2+Fe3+Mn)2(Si,P)8032 . Ba (abundant in the original analysis of the slag mineral) is not a constituent. Accurate X-ray powder data of the natural mineral are given. Bredigite is not Ca2SiO,, nor is it part of a solid solution of variable composition between larnite and merwinite. Analyses are presented for the associated minerals larnite (allowing appraisal of its composition), spurrite, and spinels. The paragenesis is discussed.
The peninsula in which the mainland of Shetland ends north of Ronas Voe and Colla Firth is occupied mainly by the Ronas Hill Granite. This body is intrusive into a metamorphic series consisting of orthogneiss on the north and paragneiss oil the north-east, east, and south-east of tile peninsula. It is a biotite-granophyre within which earlier bodies of basic rock varying from diorite to gabbro are found. It is unsheared and unfoliated and is cut by a swarm of dikes with north-south trend which extends from tile north coast to Ronas Voe. There is evidence also of the existence of dikes earlier than the granite. The later dikes comprise spessartine, microdiorite, porphyrite, quartzfelspar- porphyry, and a great variety of felsites which are generally granophyric and spherulitic. Many of the felsites contain riebeckite and aegirine and have a corresponding blue or blue-green colour. From observation of chilling of dike against dike and intersection of dike by dike it has been found that tile basic dikes, spessartine, microdiorite, and porphyrite, are the oldest of the suite, the quartz-felspar-porphyries are older than the felsites, and the blue riebeckite-felsites are younger than white, pink, and dull red felsites which contain neither aegirine nor riebeckite
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