Keele Research Repository

The Lough Guitane volcanic rocks occur interbedded with the Upper Devonian fluviatile clastic sediments of the Munster Basin. Most of the volcanic rocks are shown to be associated with 3 separate volcanic centres, and consist exclusively of thick rhyolite lavas and acid volcaniclastics. Their distribution was strongly influenced in the Bennaunmore volcanic centre by complex faulting contemporaneous with the volcanism.
The volcaniclastics are always observed mixed with some terrigenous clastic sediment, and are believed to have been erupted in this mixed state as a result of rapid vent erosion, although some water reworking also occurred. There is a notable absence of evidence for explosive volcanic activity, and the lavas are virtually free of vesiculation.
The originally glassy lavas have been completely recrystallised to a quartz-albite-chlorite-phengite assemblage, with minor quantities of allanite, anatase and iron-ore. Despite this recrystallisation textural evidence remains for the incorporation of 1-2% of restite in the magma.
Geochemical analysis of the least altered specimens provides evidence for the derivation of the subalkaline rhyolitic magma by high degrees of partial melting of an anhydrous, acidic deep crustal rocks, controlled by the decomposition of amphibole at temperatures in excess of 950°C.
Using mass-transfer computations, widely varying lava and dyke whole rock compositions are shown to have been probably derived from a common rhyolitic parental composition by varying degrees and types of metasomatism. Under severe metasomatic conditions only Zr, Ti, P and A1 can be shown to be geochemically immobile.
The area was subjected to shallow, partially brittle deformation during the Hercynian orogeny, leading to the development of E-W aligned structural features including a major anticlinorium lying south of a large reverse fault. Strong variations in the resistance to the compressive stresses due to the presence of thick lava flows and unexposed shallow intrusions led to the development of large oblique- slip cross faults which offset the major reverse fault in the north.
It is suggested that the orientation of volcanic features, and the geometry of the Munster Basin are cogenetically related by the same E-W oriented deep crustal fractures.