Dolerites - Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75
|From: Howells, M. F., Francis, E. H., Leveridge, B. E. and Evans, C. D. R. 1978 Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75 Classical areas of British geology, Institute of Geological Sciences. (London: Her Majesty's Stationery Office.)
Chapter 6 Dolerites
Intrusions of dolerite are most prominent in the northern and western parts of the district, where they cut strata ranging from Carneddau Group slates overlying the Capel Curig Volcanic Formation up to the Middle Crafnant Volcanic Formation. Most of them are in the form of sills which are locally transgressive, but there are also a few small bodies of indeterminate morphology, together with a few dykes trending in various directions.
The largest intrusion is bifurcated along the spine of Moel Siâbod to give two sills, each about 150 m thick, forming cliffs above and ridges below the cwm of Llyn-y-Foel. The southernmost sill, itself locally in two leaves, occupies an horizon within the sandstone-tuffite sequence of the Carneddau Group. It can be traced almost continuously within that sequence north-eastwards along the flank of a syncline to the Mon Llugwy (Figure 21), then back south-westwards on the opposite flank of the structure. Such distribution indicates emplacement before the main folding–a view supported by the presence of cleavage, though this tends to be local here and in other intrusions. Most intrusions clearly predate the faulting, but there is equivocal evidence to suggest that others are later than, or are controlled by, faulting, as exemplified by the intrusion south of Craig Forris [SH 757 570]. Thus it would appear that there was more than one period of either intrusion or of faulting.
At outcrop the dolerites are generally greyish green rocks with a brown massive rounded 'granitic' type of weathering. They tend to be more resistant to erosion than the sediments and thus form crags, though in places, and particularly at their margins, they are marked by peaty depressions. At many margins columnar jointing is common. Even where the sills appear from their outcrop pattern to be generally concordant with adjacent sediments and tuffs the marginal contacts are often aberrant. On the southern slopes of Moel Siâbod, for instance, well exposed upper and lower contacts change inclination within a few hundred metres along the strike, from 50°–80° to the SE, concordant with the sediments, to similar but discordant angles towards the north-west.
The extent of contact alteration appears to bear little relation to the thicknesses of the intrusions. The widest observed zone of induration and spotting is about 10 m, flanking the sill immediately to the north-east of the summit of Moel Siâbod, but this is exceptional. Similarly, dolerite textures bear little relationship to the thicknesses of intrusions. Traced inwards from narrow zones of chilling at the margins, most rocks are medium-grained, with no apparent progressive increase in coarseness. Ill-defined coarser leucocratic pockets with bladed ferromagnesian minerals may be scattered throughout, as in the sill below Llyn-y-Foel. One of the coarsest rocks, cropping out east of Caer Llugwy [SH 749 524] is gabbroic almost up to the margins, though the sill is not one of the thickest in the district.
Microscopic examination suggests that the dolerites were probably composed originally of clinopyroxene (commonly titaniferous augite), zoned labradorite and opaque titaniferous oxides. In some specimens (E35467) pseudomorphs after olivine are also prominent and in others accessory apatite is conspicuous (E38149). Textures are commonly ophitic, with feldspar prisms up to 1 mm long and 0.2 mm across (Figure 22). In the coarse, gabbroic variants (E35468) the feldspars are as large as 4.5 x 2.5 mm. Some rocks contain ocelli filled, wholly or in part, by finer grained dolerite.
All the intrusions have suffered severe deuteric alteration and some have been further modified by regional metamorphism. The most obvious deuteric effect is the total nondestructive albitisation of original labradorite to oligoclaseandesine. The original labradorite is inferred from the texture and by analogy with specimens of the same suite to the north of the district, where relict calcic plagioclase is preserved. Other deuteric effects include the growth within plagioclase, or outwards from pyroxene, of bladed to fibrous or even hairlike sheaves of actinolite; the development of leucoxene from ilmenite; and, in the final stages, the growth of chlorite and epidote followed by quartz and carbonate in intersertal spaces and ocelli.
The effects of subsequent regional metamorphism are most easily recognised in the imprint of a tectonic foliation. This is clearly indicated by a further oriented growth of chlorite, quartz and carbonate at the expense of feldspar and pyroxene, the latter being typically totally replaced. Dolerites free from pyroxene commonly contain poikiloblastic, graphic, strained quartz crystals, probably dating from the regional metamorphic phase of alteration. The first effect of foliation on the feldspars is the development of laminae; lithons of oriented chlorite grow along the laminae until only fragmented remnants of plagioclase remain in a totally modified igneous fabric (E35262).
Floyd and others (1976) have provided geochemical data on these rocks, which they describe as meta-dolerites, probably formed quite near the margin of a stable continental plate.
Tertiary dykes are reported from Snowdonia (Williams and Ramsay, 1959) and there are petrographical grounds for ascribing a Tertiary age to a dyke proved underground in a borehole in the north-eastern part of the district (Archer and Elliot, 1965). None have been found at outcrop during the survey; however, a 2.5-m dyke on Creigiau'r Garth, some 0.5 km S of Llynnau Mymbyr, has an appropriate ESE trend, but it is cleaved and highly altered.