RATES AND TEMPO OF GRANITIC MAGMATISM: A U-PB GEOCHRONOLOGICAL INVESTIGATION OF THE DONEGAL BATHOLITH (IRELAND)

The late- to post- orogenic Donegal Batholith of the north Irish Caledonides comprises a number of igneous complexes that range in composition from gabbro to granite, and have a combined onshore aerial extent of ca. 1100 km2. Several of the intrusive complexes have been the focus of intensive studies during the last century and central to the development of the understanding of magma emplacement processes. Based upon structural and petrological field mapping, emplacement mechanisms have been proposed for individual complexes indicating batholith construction by a combination of ‘forceful’ (i.e., space created due to mainly magmatic derived stresses) and/or ‘passive’ (i.e., space created due to mainly non magmatic derived stresses) processes, ranging from diapirism/ballooning (Ardara Granite) to emplacement into active shear zones (Main Donegal Granite). An outstanding question remains the rates and tempos at which such processes operate.

New high-precision isotope dilution thermal ionization mass spectrometry U-Pb zircon and monazite geochronology for all of the plutons that comprise the Donegal Batholith indicate punctuated emplacement between ca. 428 and 400 Ma. A geochronologically constrained intrusion sequence is consistent with field relationships and has also revealed close temporal associations between apparently disparate intrusive suites as well as links with co-existing normal faults external to the batholith. Geochronological data from sub-plutonic scale units (i.e., individual sheets, concentric zones etc.) allows for assessment of the duration and rates for construction of individual plutonic complexes as well as time integrated strain rates for end-member accommodation processes. For example, data from the Ardara Granite necessitate rapid forceful emplacement (< 1 Myr) of the concentrically zoned pluton and associated mafic magmas. In contrast, ages obtained on Main Donegal Granite units indicate protracted magmatism (≥ 7 Myr) contemporaneous with sinistral shear. These end member examples illustrate the contrasting rates and tempos of ‘forceful’ and ‘passive’ emplacement processes reflecting the various processes (such at local tectonics, magma supply etc.) operating during construction of the Donegal Batholith.