VIII.—The Petrology of the Layered Basic Rocks of the Carlingford Complex, Co. Louth

Abstract

The principal basic components of the Tertiary Carlingford complex are believed to have formed through the differentiation of magma of a high-alumina basaltic type. These rocks, which were once thought to be in the form of a ring-dyke, are shown to occur as a floored and multi-layered intrusion. Four major layers have been recognized. Within each, differentiation through crystal fractions settling under the influence of gravity is believed to have been the chief factor producing chemical variation. The effect of this process is seen in the compositional and modal variations of the gabbroic rocks and their constituent minerals. The variations from bottom to top of each layer correspond to a decreasing temperature of crystallization with height. At the bottoms are gabbros enriched in olivine. At the tops, the gabbros are rich in plagioclase. Throughout the layers, the plagioclases are transitional between the high and low (temperature) structural states. The resemblance of these rocks to other layered intrusions (e.g. Rhum, Bushveld, etc.) is taken as proof of the magmatic origin of the Carlingford layered basic rocks. The base of the lowest layer is in contact with Silurian sediments and Tertiary basalts. The sediments have been intensely metasomatized with reciprocal contamination of the gabbro immediately above. Greater extremes of metasomatism and mobilization are seen in the sedimentary xenoliths which occur within the gabbros. At the base of the topmost layer of gabbro, a horizontal screen of highly metasomatized Silurian sediments separates that gabbro from the one below.