A NOTE ON CARBONATE-FILLED NEPTUNEAN DYKES IN THE OX MOUNTAINS GRANODIORITE, NW IRELAND

Abstract

This paper documents the first record of carbonate-filled neptunean dykes within the Ox Mountains Granodiorite. A post-Carboniferous age for the formation of the dykes is rejected and a Visean age favoured. Differential uplift and tilting of segments of the Ox Mountains inlier could have contributed to a complex sub-Visean unconformity and Visean deposition. Field relationships and petrology of the neptunean dykes Neptunean dykes within the Ox Mountains Grano diorite crop out on a peninsula of the isthmus near Pontoon (G 209 005) between Lough Conn and Lough Cullin, Co. Mayo (Fig. 1) at a height of 15m above sea level. They are formed mainly of horizon tally laminated micritic limestones. Seven neptunean dykes occupy eroded vertical joints with a mean ori entation of 098? (Taylor 1966, Figs 32A and 32D; Crane 1984. Fig. 32), extend to a depth of less than a metre, and are less than 2cm in width (Figs 2a and 2b). No evidence of movement along the contact walls has been observed. The dyke infills have sharp contacts with the granodiorite (Figs 2a-2c) and con sist of alternating clean light grey (N7, Geological Society of America 1970) and silty dark grey (N3) sub-parallel bands that are usually less than 3mm in thickness (Fig. 2b). Some bands exhibit small-scale load/erosional structures (Fig. 2b). Since the bands are approximately horizontal they are probably in their formation position. The mineralogy of the infills is predominately micro crystalline calcite (>96%) with clastic fragments of main ly silt-sized sub-angular quartz (rarely exceeding 0.5mm in diameter) and rare altered feldspar and micas (Fig. 2c). Most likely the clastic fragments have been derived from the adjacent granodiorite. Palaeontological material has not been observed and attempts to isolate microfos sils from the infill have proved unsuccessful. Hence, the age of formation for the neptunean dykes has not been resolved with any certainty.