Isotopic dating of overthrusting, collapse and related granitoid intrusion in the Grampian orogenic belt, northwestern Ireland

Abstract

In the Grampian (Ordovician) orogenic belt in northwestern Ireland the Dalradian Supergroup abuts and interleaves with an enigmatic psammite-dominant unit, with relict latest Precambrian granulite-facies assemblages, known as the Slishwood Division. These two units were most likely tectonically juxtaposed during the D₃ phase of Dalradian deformation. Using Ar–Ar, Rb–Sr and Sm–Nd methods, 53 new mineral ages have been obtained from both rock units, and from pegmatites intruding them, in an attempt to constrain the timing of their juxtaposition and the history they subsequently shared. Prior to D₃ (c. 480 Ma), retrograde hornblende poikiloblasts grew in granulite-facies metabasite pods within the Slishwood Division. Tectonic juxtaposition (D₃) with the Dalradian is loosely constrained between 479 Ma and 459 Ma but is likely to have occurred between 470 Ma and 459 Ma. Dalradian peak metamorphic conditions were attained shortly after D₃, at around 460 Ma. Extensional collapse of the orogen, with rapid uplift and exhumation, is dated by an abundance of mineral cooling ages between 460 Ma and 450 Ma. Orogenic collapse is also dated, more precisely, by widespread pegmatite intrusion into both rock units at about 455 Ma. Initial ⁸⁷Sr/⁸⁶Sr ratios suggest that these pegmatites are partial melts from Dalradian metasediments. A second suite of pegmatites were intruded, along with the Ox Mountains Granodiorite, much later at or around 400 Ma during sinistral shearing.