Connemara Dalradian Formation, western Ireland: implications for the A local source for the Ordovician Derryveeny

Facies and palaeocurrent data indicate that the conglomeratic Ordovician Derryveeny Formation is the deposit of an alluvial fan whose apex was located a few kilometres east of the present outcrop. Clasts in the conglomerates include migmatite, schist, gneiss, granite, acid porphyry, spilite and vein quartz. Sillimanite bearing migmatite clasts closely resemble rocks of the neighbouring Lough Kilbride Formation, a unit of the Connemara Dalradian (Lower Proterozoic) basement from which they are presumed to be derived. This local source for the conglomerates is also suggested by the similarity of the Nd-model ages (c. 2.15 Ga) from the metamorphic clasts and from the Lough Kilbride Formation. Moreover peraluminous granite clasts give the same model ages, suggesting the granites are intracrustal melts from the same source. Interpretation of the Connemara Dalradian as a suspect terrane requires that it docked before Upper Ordovician times. Differences between the clast assemblage in the Derryveeny Formation and the presently exposed Lough Kilbride Formation are related to stripping of the original cover, emphasizing the value of clastic detritus in the study of the uplift history of metamorphic basement. Conglomerates are strongly favoured in provenance studies, because they are generally the least travelled clastic detritus and they provide the largest samples of the ambient source rocks. Yet, because of the very existence of these conglomerates, the source area of the clasts must have changed in character during uplift and erosion such that exact matches between clasts and source are rarely achieved. With care, differences between clasts and what is presently exposed in the inferred source area can be reasonably interpreted and linkages established. Conversely, clastic detritus is an important source of evidence in studying the uplift history of metamorphic basements for which the present exposures yield only an incomplete history. This study focuses on part of the northwest (Laurentian) margin of the Caledonides which is generally agreed to have been an active, destructive continental margin during Cambrian and Ordovician times. In Britain and Ireland the late Precambrian basement rocks of this margin are termed the Dalradian Supergroup (Harris & Pitcher 1975). These always lie to the northwest of demonstrable Lower Palaeozoic rocks except in South Mayo, western Ireland, where the Dalradian rocks of Connemara lie to the south of extensive Lower Palaeozoic outcrop (Fig. 1). The Connemara Dalradian rocks are unusual both in their anomalous position, significantly east of where they might be expected f r o m along-strike interpolation (Leake et al. 1984), Fig. 1. General location and outline geology of the British Isles Caledonides. GGF, Great Glen Fault; HBF, Highland Boundary Fault; iS, Iapetus Suture; MTZ, Moine Thrust Zone; SMT, South Mayo Trough; SUF, Southern Uplands Fault. From Morton, A. C., Todd, S. P. & Haughton, P. D. W. (eds), 1991, Developments in Sedimentary Provenance Studies. Geological Society Special Publication No. 57, pp. 199-213. 199 2012 at University College Dublin on December 10, http://sp.lyellcollection.org/ Downloaded from