Controls on magmatism in the Munster Basin, SW Ireland

Abstract

The following paper is intended as a review of the magmatism in the Munster Basin and an initial attempt to link it with the basin’s history. During the initiation, evolution and inversion of the Devono-Carboniferous Munster Basin magmatic activity was widespread, although small in volume. The episodic occurrence and diversity of the magmatism gives insights into the basement involvement and structural controls relating to the basin’s history. The differences in composition, location and structural relationship of the magmatic bodies are used as indicators for the timing of their emplacement and their relationship to the basin’s evolution. On the western Beara Peninsula in SW Ireland alone, more than 160 sheet-like intrusions, a wide variety of tuff bands and a deep-seated pipe-like structure of lamprophyric affinity occur. Other centres of magmatism in the basin are the Lough Guitane area, where rhyolitic lava flows and acidic pyroclastic rocks are associated with contemporaneous basin faults, and the Valentia Harbour area, where doleritic sills are associated with a volcanic breccia. A rhyodacitic tuff can be traced as far as St Finan’s Bay on the western shore of the Iveragh Peninsula. The chemical composition of the extrusive and intrusive magmatic bodies ranges from tholeiitic dolerites (Valentia Harbour) and subalkaline rhyolites (Lough Guitane) in Mid-Devonian time to subalkaline basalts and tuffs (e.g. Beara Peninsula) in Late Devonian time. In Late Carboniferous and possibly Permian time alkali basalts, trachytes and phonolites (e.g. Beara Peninsula) occur. The igneous activity in the Munster Basin is linked to the basin’s history by the interaction between active faults and fractures opening up during various stages of stress imposed on the basin and exploitation of these faults and fractures by rising magma.

The vein-hosted copper deposits of the Allihies mining area, SW Ireland; a new structural and chronological evaluation

“…A series of minor sheet-like intrusions of mainly alkali basalts, trachytes and phonolites have been found along the northern coastline of Beara Peninsula (Pracht 2000;Fig. 1).…”

“…These basalts are interpreted as surficial intrusions during the subsidence phase of the Munster Basin. Pipe-like lamprophyric intrusions dated at 314.44 ± 1.00 Ma (Ar/Ar phlogopite, Quinn et al 2005) occur at Black Ball Head (5.6 km south of Allihies), which were formed during the early Variscan compression (Pracht & Kinnaird 1995;Pracht 2000).…”