Contributions to the Petrology of Barnavave, Carlingford, I.F.S. 3: On some Hybrids from the E. and S.E. Slopes of Barnavave Mountain

Extract

The rocks about to be described are found in various parts of the eastern and south-eastern portions of the eucrite mass which forms the bulk of Barnavave Mountain. They are best displayed, however, between the 700 and 800 foot contours in the area just north of the large N.W. fault which crosses the region.¹ The main characters of the normal eucrite are already well known from Sollas's now classical researches on the western side of the mountain.² The normal eucrite of the eastern part of Barnavave is very similar to that occurring in the west. It is composed essentially of a diallagic pyroxene and a basic plagioclase, with or without a small but variable quantity of rhombic pyroxene (Text-fig. 1A). The plagioclase varies from Ab₁₉An₈₁ to Ab₃₀An₇₀, but the bulk centres round the more acid of these extremes. When subjected to metamorphism by the later acid intrusions, it shows very fine “clouding” and may appear black in hand specimen. The diallagic pyroxene is almost colourless in thin section but has a pale brown colour in thicker pieces. It has a % 1.680 ±.003, γ % 1.703 ±.003, γΛc % 41°, (+) 2V % 63°±4°, dispersion moderate p>v. Achemical analysis will be found on p. 477. In places it has a well defined schiller structure. The rhombic pyroxene (enstatite), when present, is colourless and non-pleochroic in thin section and also shows schiller structure. The accessory constituents include some titaniferous magnetite or ilmenite (normally enclosed in pyroxene). Apatite appears to be negligible in quantity.