Zonation and structure of the Late Caledonian Newry Igneous Complex, Northern Ireland from Tellus geophysical datasets [abstract]

Abstract

The Late Caledonian Newry Igneous Complex was intruded into the Southern Uplands-Down-Longford Terrane just after final closure of Iapetus (c. 425 Ma). Work to date has shown it to be composed of three overlapping, I-type, granodioritic plutons and an intermediate-ultramafic body. The intrusions are aligned and young from NE-SW. Petrographical and geochemical studies have shown the NE and SW plutons to be normally zoned (more basic at margins) and the central pluton reversely zoned, however to date no mapped internal division of the plutons has been published. Interpretation of high resolution geophysical imagery, from the Geological Survey of Northern Ireland Tellus project, has allowed internal zonation and structures not previously described for the complex to be mapped. Ternary radiometric data show the central pluton and the south-western part of the NE pluton to be relatively rich in potassium, compared to the north-eastern area of the NE pluton which is thorium elevated. Occurring within this thorium elevated zone is the intermediate-ultramafic body and a similar mass in the vicinity of Kilcoo near Lough Island Reavy, which show a mixed potassium/thorium signal. Magnetic data reveal striking magnetite enriched rings inside the central and NE plutons, and also define the intermediateultramafic bodies. There are also two circular, parallel structures visible on the magnetic image. The inner defines the outer edge of the magnetic ring of the NE pluton, while the other, outer structure cuts the country rock and north-eastern part of NE pluton. Associated with this structure are outlying granodiorite masses such as that mapped at Ballynafern ~ 6 km NNE of Rathfriland