Scottish Dalradian Research Articles

The Green Beds of the Scottish Dalradian Series: Geochemistry, Origin, and Metamorphism of Mafic Sediments

The Green Beds of the Scottish Dalradian Series are greenschists and amphibolites of metasedimentary origin. Field relations, petrography, and chemical data are used to demonstrate that these rocks, approximately of basaltic composition, were derived by metamorphism of basic tuffs deposited in an area of pelite sedimentation. The Dalradian Epidiprites, a series of orthoamphibolites associated with the Green Beds are less siliceous and less magnesian, in general, than the metasediments. Pelitic material is more abundant in the Green Beds of the Southwest Highlands than to the northeast. Amphibole occurrence and abundance are related to calcium content of the rocks. Fifty-eight metasediments and three Epidiorites were analyzed for thirty elements each, by X-ray fluorescence methods.

The clay mineralogy of the Scottish Dalradian meta-limestones

The clay mineralogy of the Scottish Dalradian meta-limestones has been studied mainly by X-ray diffraction. In the samples from the north-east part of the Dalradian outcrop, which often show evidence of intense metamorphism, clay minerals found include saponite, regularly and irregularly interstratified smectite-chlorite, swelling chlorite, normal chlorite, illite and kaolinite. Limestones from the weakly metamorphosed South-West Highlands area contain only kaolinite, illite and chlorite. The genesis of the clay minerals is considered, particular attention being paid to the swelling minerals from the north-east region.

The structure and stratigraphy of the Dalradian rocks of north Achill Island, County Mayo, Ireland

Achill Island is underlain by a mixed suite of low-grade metamorphic rocks. The lithological units of the northern part, the subject of this paper, are systematically described. The deformation history can be divided into five separate phases. The first phase, D 1, produced isoclinal F 1 folds and S 1 flattening schistosity in mica schists; and an L -tectonite fabric associated with some tight F 1 folds in psammitic rocks. The second phase, D 2, produced tight to isoclinal reclined F 2 folds and S 2 strain-slip schistosity in mica schists; and S 2 flattening schistosity or an L–S fabric in psammitic rocks. This was followed by the third phase, D 3, which produced tight to close upright folds and S 3 strain-slip schistosity. The folds formed during these phases plunge gently between north-east and south-east. This variation in the trend of the plunge is the result of open F 4 refolding. Finally, the latest deformation, D 5, resulted in the development of kink bands. Three tectonic slides, of D 1 age, are recognized in the area, one of which separates rocks of two separate stratigraphical successions. This slide can be mapped around the major southward-facing F 2, folds of the area. One large dextral strike-slip fault is present. The two stratigraphical successions are compared with the Scottish Dalradian; one with the Islay Succession, and the other with the Moine Series. In North Achill large-scale F 2 folding places the equivalents of the Moine Series structurally above the Islay succession in the core of an F 2. anticline.

The geochronology of the Connemara granites and its bearing on the antiquity of the Dalradian Series

Thirty-eight whole-rock samples of Connemara granites give the following Rb–Sr whole-rock ages: Galway granite, 384 ± 1 m.y.; Omey granite, 3884 ± 17m.y.; Roundstone granite, 3954 ± 80m.y.; Inish granite, 404 ± 8m.y.; Oughterard granite, 510 ± 35 m.y.; potash-feldspar gneisses, 725 ± 175 m.y. As the field evidence shows that the Oughterard granite was intruded into the Dalradian metasediments post- F 4 in the Connemara fold-sequence, and possibly post- F 5 , this dates the fold phases F 1 to F 4 and the metamorphisms M 5 to M 1 as pre-510 ± 35m.y. The older isotopic age of the potash-feldspar gneisses agrees with their field-deduced age of post- F 2 pre- F 3 . Muscovite, potash feldspar, and whole-rock Rb–Sr analyses show that the Oughterard granite was metamorphosed about 444 ± 7 m.y. ago, which is close to the K–Ar age obtained from three hornblendes separated from potash-feldspar gneiss. These results indicate that the post-500 m.y. dates obtained from the Connemara Dalradian rocks, and therefore presumably the Scottish Dalradian also, are the dates of late metamorphisms. While the possibility of Pre-Cambrian metamorphism and folding in the Dalradian is apparent, the results are equally in agreement with a Cambrian age for the main metamorphisms M 1 to M 5 and foldings F 1 to F 4 , especially in view of the present uncertainty about the age of the Cambrian period. The base of the Arenig would seem to be younger than 510 ± 35m.y.

Some mineralogical and chemical changes induced by progressive metamorphism in the Green Bed group of the Scottish Dalradian

The petrology of the igneous rocks has for the last quarter of a century been established,on a sound quantitative basis, largely as a result of the body of experimental work available on silicate melts. In the field of metamorphism, however, progress has been much less marked, and the difficulties of the laboratory development of stresses comparable with those acting during regional metamorphism must for some time leave our knowledge of the crystalline schists far behind that even of the hornfelses. A quantitative approach, however, is not confined to the experimental side. The introduction of the conceptions of facies and grade raises a new need for exact knowledge of the compositions of the various minerals, both of those constituting the different isophysical assemblages of a given grade, and of the isochemical assemblages representing a given rock under the grade-conditions of the various facies established.