Constraining Sinistral Shearing in NW Ireland: A Precise U-Pb Zircon Crystallisation Age for the Ox Mountains Granodiorite

Abstract

Abstract:The age of the Ox Mountains Granodiorite has proved controversial in the past. Two published Rb-Sr whole rock isochrons suggest ages of $487 \pm 6 Ma$ and 500 $\pm 18 Ma$, respectively ($\lambda^{87}Rb = 0.0139 Ga^{-1}$), whereas Rb-Sr muscovite-feldspar isochrons from undeformed pegmatites associated with the intrusion have yielded c. 400Ma ages. The syn-kinematic nature of the Ox Mountains Granodiorite has been clearly demonstrated by several workers, with the pluton being emplaced as a series of sheets in a major transpressive sinistral shear zone. Knowledge of the age of crystallisation of the Ox Mountains Granodiorite would thus constrain the timing of strike-slip movement along this shear zone, which forms part of a major Caledonide structure known as the Fair Head - Clew Bay Line. A U-Pb zircon age (conventional isotope dilution TIMS) of $412.3 \pm 0.8 Ma$ from a tonalitic facies of the pluton is presented here. The age confirms that emplacement was broadly contemporaneous with the majority of other volumetrically important Irish granite batholiths (i.e. Leinster, Donegal, Galway and Newry), while $\inHf_{(412)}$ values of the dated zircons (which range from -5.4 to -7.7) imply incorporation of old crustal material into the parent melt.