About This Item
Share This Item
Provenance of Oligocene Lough Neagh Group, Northern Ireland, United Kingdom: Abstract
The Oligocene Lough Neagh Group in Northern Ireland consists of three lithological units. The lowermost unit is weathered basalt and clay-rich conglomerate overlain by an alternating sequence of clays and lignite. The clays contain silt and sand laminae and in places contain fragments of basaltic, micritic, rhyolitic and schistose rocks. A detailed petrographic analysis of the heavy minerals in the Lough Neagh sediments and possible source rocks has been carried out to determine the provenance of the sediments.
The heavy minerals identified in this study indicate that the Lough Neagh sediments were derived from several sources. Five significant heavy mineral suites are evident which are interpreted as: 1) acid igneous, 2) basic and ultrabasic igneous, 3) pegmatitic, 4) high grade metamorphic, and 5) sedimentary sources. Three of these suites can be matched with heavy minerals in the Dalradian (Precambrian) metasediments, the Ordovician Tyrone Igneous Complex and Tertiary basaltic lavas.
In addition to the mineral suites, the presence/absence of zoning and inclusions in zircon grains, their shape, colour and chemical composition indicate particular members of the Tyrone Igneous Complex as source rocks. Variation in the elemental composition of zircon and opaque minerals was used to help determine the provenance of sediments. For example, zircons from biotite-granodiorite contain a very high percentage of Ta2O5, while the zircons from Tyrone granite contain ThO2, UO2, Ta2O4, WO3, and SrO2 as minor oxides. Ilmenite grains from Tyrone granite contain various minor oxides such as BaO and Ta2O5 etc., whereas ilmenite grains from Tertiary basalts generally do not contain any minor oxides (except CuO).
It is also possible to divide the Lough Neagh Group into several stratigraphic horizons on the basis of lithology, heavy mineral suites, and chemical composition of zircons and opaque mineral grains.
Acknowledgments and Associated Footnotes
1 Department of Geology, The Queen’s University of Belfast, Belfast BT7 1NN, Northern Ireland
Copyright © 2009 by the Canadian Society of Petroleum Geologists