Structural and stratigraphic relationships across the continuation of the Highland Boundary Fault in western Ireland

Abstract

The relationship between the Dalradian Supergroup and the Highland Border Complex in Scotland has remained contentious for over a century. In western Ireland, the contact between the Dalradian Supergroup and the Clew Bay Complex (a correlative of the Highland Border Complex) is superbly exposed on the island of Achill Beg on the North Mayo coast. The unfossiliferous South Achill Beg succession has been traditionally assigned to the Clew Bay Complex, and this interpretation is supported by a combination of Sm-Nd model age data, heavy mineral analysis and lithostratigraphic correlation. T DM ages range from 1.99-2.66 Ga (mean = 2.28 Ga, n = 6). Detailed structural mapping shows that both the Dalradian and the Clew Bay Complex share the same structural history. A D1 high strain event is common to both units, and is associated with the development of tectonic slides. The D2 event is responsible for the formation of crustal-scale nappes. In both units, beds are consistently downward facing on the S2 foliation. Later dextral shearing (D3) resulted in the tilting of the originally recumbent, S-facing D2 nappes into this downward-facing orientation. Rb-Sr and 40 Ar-39 Ar radiometric dating of muscovite confirms that both units were deformed contemporaneously as the S2 nappe fabric in each is dated at c. 460 Ma. This Middle Ordovician age for deformation of the Clew Bay Complex is highly significant, not least because published microfossil data suggest a Silurian age.

Laurentian crustal recycling in the Ordovician Grampian Orogeny: Nd isotopic evidence from western Ireland

“…Complex metasediments show that the Killadangan and Ballytoohy formations, which crop out on the southern shore of Clew Bay and on Clare Island, respectively , have similar Nd composition to the Clew Bay Complex on Achillbeg (Chew, 2003). Clew Bay Complex samples are in general slightly more negative than the Dalradian averages, with a range from ∼ − 13 to − 24 and median and mean values between − 15.9 and − 19.4 (Table 2; Harkin et al 1996;Chew, 2003).…”

“…Clew Bay Complex samples are in general slightly more negative than the Dalradian averages, with a range from ∼ − 13 to − 24 and median and mean values between − 15.9 and − 19.4 (Table 2; Harkin et al 1996;Chew, 2003). Because samples from the South Mayo Trough have yielded ε Nd(t) values below − 14, the maximum average ε Nd(t) value of the continental crust involved in petrogenesis must be lower than this.…”

“…Ph.D. thesis, University Coll. Dublin, 2001;Chew, 2003). Lithologically, the Clew Bay Complex indicates sedimentary derivation from a passive continental margin and is most reasonably interpreted as the Ordovician accretionary prism to the N-facing Lough Nafooey Arc, composed largely of detritus from the Laurentian continent (Dewey & Ryan, 1990;Dewey & Mange, 1999).…”

“…The North Mayo Dalradian sequence includes the late Proterozoic Grampian, Appin and Argyll groups, which consist of, respectively, schists and psammites, quartzites derived from mature sands overlain by interbedded shales and carbonates, and quartzites interspersed with metabasic volcanic rocks (Winchester & Max, 1996). SE-verging nappes within these Dalradian units in North Mayo, the Sperrin Mountains and the Ox Mountains inlier, have been shown to have originated during Grampian deformation, as the Dalradian sequence was overthrust over volcanic units and/or basement probably associated with the colliding Lough Nafooey Arc (Alsop & Hutton, 1993;Flowerdew et al 2000;Chew, 2003).…”

“…We assume that Nd is approximately seven times more concentrated in Laurentian crust than in the primitive arc melt. This assumption is based upon REE analyses from North Mayo Dalradian and Clew Bay Complex samples (Menuge & Daly, 1990;Harkin et al 1996;Chew, 2003) and from 21 analyses of the Bohaun Volcanic Group, a combination of the two analysed for this study and 19 presented by Clift & Ryan (1994). The ε Nd(t) value that would result from mixing melt of these two compositions is calculated using the relationship:…”