HEAVY MINERAL SIGNATURE OF THE ORDOVICIAN GRAMPIAN ARC/CONTINENT COLLISION IN THE CALEDONIDES OF WESTERN IRELAND

In the western Irish Caledonides, we can make a fine-scale correlation between structural/metamorphic events in the Grampian Orogen, the tectonic events that caused them, and the detrital mineralogy of the synchronously-deposited Ordovician strata of the South Mayo Trough (SMT). During the Tremadoc and early Arenig, an oceanic arc (Lough Nafooey) faced towards the Laurentian rifted margin, at which the Neo-Proterozoic to Early Ordovician Dalradian Sequence had been deposited. The Killadangan was a subduction-accretion prism along the northern margin of the arc; it is dominated by rounded purple zircons of probable Grenville origin. The Letterbrock is roughly equivalent to the Lough Nafooey and contains Killadangan and ophiolitic detritus. As the arc approached the rifted margin, it became silicic, reflected in doubly-terminated clear zircon in the Sheeffry. Northward obduction and erosional denudation of the arc is recorded by basaltic/gabbroic detritus in the Letterbrock and Derrymore to chromite in the Sheeffry and Derrylea. D1-D3, north-vergent, Grampian deformation and metamorphism occurred during this obduction, much of which was below sea-level; blueschists developed in the highest Dalradian. In the early Llanvirn, a flood of almandine, staurolite, metamorphic zircon, and muscovite came from the north into the SMT Upper Derrylea, suggesting rapid extensional denudation, probably generated by extensional collapse of the Grampian core, enhanced by subduction roll-back following subduction polarity flip. Extension continued and led to reduced relief, with the pelitic Glenummera and its muted heavy mineral population. This phase of extensional denudation, on low-angle detachments, denuded the Grampian metamorphic core, exhumed blueschists, and preserved the Killadangan accretionary prism and the SMT. In the mid-Llanvirn, shortening in the post-flip arc resulted in the D4 south-vergent Mannin Thrust, the Connemara Antiform, tightening of the SMT, erosional denudation to generate the fluvial Mweelrea, and a flood of Grampian heavy minerals. Along the southern margin of the post-flip arc, northward subduction is recorded in the South Connemara and Southern Uplands (SUAC) accretionary complexes. These contain abundant Grampian detritus but the SUAC also contains glaucophane and lawsonite.