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

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 ²⁰⁷Pb/²⁰⁶Pb zircon age of 467 ± 12 Ma whereas he 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.