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

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 8C, 6.8 kbar) and leucosomes in paragneisses are cut by granite pegmatites, which post-date two deformation fabrics. The leucosomes yield a weighted average 207Pb/206Pb zircon age of 467 � 12 Ma whereas the main fabric yields a 40Ar- 39Ar biotite cooling age of 468 � 1.4 Ma. The pegmatites yield 457 � 7 Ma and 458 � 7 Ma Rb-Sr muscovite-feldspar ages and 40Ar- 39Ar 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 (¼ 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 geochrono- logical studies relating to this orogenic episode have empha- sized the role that microcontinental indenters play in arc- continent 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 metasedi- mentary 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 orogen- esis. 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