The petrology and geochemistry of the tertiary dyke swarm associated with the Mourne mountain granites, Northern Ireland

Abstract

The Tertiary dyke-swarm of the Mourne area consists principally of basic and intermediate dykes and an inclined, composite cone-sheet surrounding the Mourne Mountain granites. The basic dykes are of two types: alkali basaltic dykes on the one hand, and the tholeiitic on the other. The alkali basaltic dykes make their appearances in the northern part of the swarm, while the tholeiitic types are mainly confined to the middle and southern parts. The tholeiitic series of the Mourne dyke-swarm form a differentiated sequence from olivine-tholeiites to granophyres, containing augite and sparse Ca-poor pyroxene and olivine in the basic rocks. Plagioclase is the sole feldspar phase in these rocks. The differentiated rocks of this series are typified by iron-enrichment (to tholeiitic andesites - "icelandites"). There are many textural variations within the rocks of the Mourne tholeiitic series, from medium-grained ophitic dolerites to very fine-grained spherulitic variolites and tachylytes. The origin of the basaltic parent magma and mechanism of differentiation are discussed. The ratios of various chemically determined oxides and trace element distributions indicate possible measures of parent-daughter relationships of rocks believed to represent magmas in the Mourne dyke-swarm. The chemistry of the alkali basalts of the Mourne area is related to magma generation at greater depths than in the case of tholeiitic basalts of the same swarm. A correlation is made between the alkali basalts of the Mourne area and those of the Killough-Ardglass dyke-swarm. The simultaneous intrusion of acid and basic magmas and the nature of the composite minor intrusions in the Mourne area are discussed; and a geochemical difference between the cone-sheet granophyres and the Eastern Mourne granites is demonstrated. The presence of sparse gabbroic xenoliths in the basic dykes of the Mourne area is related to a deep-seated gabbro formation beneath the Mourne Mountain granites and supports the geophysical evidence (Cook and Murphy, 1952). The other acid xenoliths contained in the intermediate dykes of the Mourne swarm are also described in detail.