470 Ma granitoid magmatism associated with the Grampian Orogeny in the Slishwood Division, NW Ireland

Ion-microprobe U-Pb zircon geochronology demonstrates that magmatic rocks that cut the Slishwood Division are early Ordovician in age and relate to the earliest stages of the Grampian Orogeny. These tonalite and granite intrusions yield ages of 474 ± 5 Ma, 472 ± 6 Ma, 471 ± 5 Ma and 467 ± 6 Ma. A Sm-Nd mineral isochron of 457 ± 36 Ma confirms an Ordovician age for one of the tonalite bodies and demonstrates that they were affected by Ordovician metamorphism whereas biotite Rb-Sr mineral ages from two of the tonalite bodies show that the Slishwood Division had cooled below c. 350 °C at 449 ± 7 Ma. The early orogenic tonalite and granite intrusions are mylonitized in structures that overthrust the Dalradian Supergroup onto the Slishwood Division during major nappe formation and, thus, date the onset of such deformation at or before 471 + 5 Ma, the age of the youngest intrusion where such field relations can be demonstrated.