tectonothermal evolution and provenance of the Tyrone Central Inlier, Ireland: Grampian imbrication of an outboard Laurentian microcontinent?, The

Journal of the Geological Society, May 2008 by Chew, D M, Flowerdew, M J, Page, L M, Crowley, Q G, Daly, J S, Cooper, M, Whitehouse, M J

Abstract:

The Tyrone Central Inlier is a metamorphic terrane of uncertain affinity situated outboard of the main Dalradian outcrop (south of the Fair Head-Clew Bay Line) and could represent sub-arc basement to part of the enigmatic Midland Valley Terrane. Using a combination of isotopic, structural and petrographic evidence, the tectonothermal evolution of the Tyrone Central Inlier was investigated. Sillimanite-bearing metamorphic assemblages (c. 670 �C, 6.8 kbar) and leucosomes in paragneisses are cut by granite pegmatites, which post-date two deformation fabrics. The leucosomes yield a weighted average ^sup 207^Pb/^sup 206^Pb zircon age of 467 � 12 Ma whereas the main fabric yields a ^sup 40^Ar-^sup 39^Ar biotite cooling age of 468 � 1.4 Ma. The pegmatites yield 457 � 7 Ma and 458 � 7 Ma Rb-Sr muscovite-feldspar ages and ^sup 40^Ar-^sup 39^Ar step-heating plateaux of 466 � 1 Ma and 468 � 1 Ma, respectively. The metasedimentary rocks yield Palaeoproterozoic Sm-Nd model ages and laser ablation inductively coupled plasma mass spectrometry detrital zircon U-Pb analyses from a psammitic gneiss yield age populations at 1.05-1.2, 1.5, 1.8, 2.7 and 3.1 Ga. Combined, these data permit correlation of the Tyrone Central Inlier with either the Argyll or the Southern Highland Group of the Dalradian Supergroup. The inlier was thus part of Laurentia onto which the Tyrone ophiolite was obducted.

The Grampian (= Taconian) orogeny resulted from the Early to Middle Ordovician collision of the Laurentian passive margin with an outboard volcanic arc terrane and an associated suprasubduction-zone ophiolite (e.g. Williams & Stevens 1974; Dewey & Shackleton 1984). Recent structural and geochronological studies relating to this orogenic episode have emphasized the role that microcontinental indenters play in arccontinent collisions. Examples include the Slishwood Division in western Ireland (Flowerdew et al. 2005), the Dashwoods block in Newfoundland (Waldron & van Staal 2001; Cawood et al. 2001) and elements of the Helgeland nappe complex in central Norway (Yoshinobu et al. 2002). In each of these examples, the microcontinental indenter was incorporated into an outboard arc terrane prior to final accretion onto the Laurentian margin.

In this study, the provenance and tectonic evolution of the Tyrone Central Inlier (Hartley 1933), a high-grade metasedimentary terrane of hitherto uncertain affinity, is investigated using a combination of petrography, mineral chemical analyses and isotopic evidence. We suggest that it may represent an outboard segment of the Laurentian passive margin, which was incorporated into an outboard volcanic arc terrane prior to accretion onto the Laurentian margin during Grampian orogenesis. This model could also imply that the Tyrone Central Inlier represents sub-arc basement to the along-strike continuation of this volcanic arc terrane, the cryptic Midland Valley Terrane of Scotland.

The Grampian orogeny and the Midland Valley Terrane in Ireland

The orthotectonic Caledonides (Dewey 1969) of Scotland consist of a series of pervasively deformed metasedimentary rocks (the Moine and Dalradian Supergroups; Fig. 1a) and associated basement inliers. The Moine Supergroup and the lower portions of the Dalradian Supergroup have been interpreted as Neoproterozoic intracratonic rift basins (e.g. Dalziel & Soper 2001) or successor basins to the Grenville orogen (Kirkland et al. 2007), with the younger parts of the Dalradian sequence recording the transition to sedimentation on the Laurentian passive continental margin (Dewey 1969). The orogenic episode affecting the orthotectonic Caledonides was termed the Grampian orogeny by Lambert & McKerrow (1976). This orogenic event is now accepted as Early Ordovician in age (e.g. Soper et al. 1999), with peak metamorphism dated at 470 Ma (Oliver et al. 2000; Friedrich et al. 1999a).

The Grampian orogeny is thought to have resulted from the Early Ordovician collision of the rifted Laurentian margin with an outboard arc terrane located to the present-day SE (e.g. Dewey & Shackleton 1984; Van Staal et al. 1998; Dewey & Mange 1999). In Scotland, the suture between the deformed Laurentian margin and the colliding arc terrane is sharply defined by the Highland Boundary Fault (Fig. 1a), which separates Dalradian Supergroup rocks from Late Palaeozoic rocks to the SE. Ophiolitic rocks of the Ordovician Highland Border Complex crop out as a series of poorly exposed fault-bound slivers within this fault zone (Tanner & Sutherland 2007). The continuation of the Highland Boundary Fault in Ireland is referred to as the Fair Head-Clew Bay Line and is defined by a conspicuous magnetic lineation (Max & Riddihough 1975), which runs from Fair Head in northeastern Ireland to the north shore of Clew Bay on the west coast (Fig. 1b). The major surface expression is a fault zone that in general lies about 10 km to the south of the magnetic lineament (Fig. 1b). The fault zone generally separates the Dalradian from the Irish correlative of the Highland Border Complex (the Clew Bay Complex) and an outboard volcanic arc terrane to the SE. The outboard volcanic arc terrane is represented by the Tyrone Igneous Complex in the central part of the north of Ireland, and by the Lower Ordovician Lough Nafooey Group and its associated forearc fill, the Lower to Middle Ordovician Murrisk Group of the South Mayo Trough in western Ireland (Fig. 1b). Additionally, unlike the Scottish orthotectonic Caledonides, high-grade polyphase deformed metamorphic rocks crop out to the SE (i.e. outboard) of the main belt of Dalradian outcrop (Fig. 1b).