Initiation and duration of Grampian orogenesis constrained by refined Sm–Nd garnet geochronology of the Ballantrae ophiolite, Scotland

Abstract

We report the results of Sm–Nd garnet geochronology on a mafic granulite from the metamorphic sole of the Ballantrae Ophiolite, Scotland. The resultant age of 477.6 ± 1.9 Ma (2σ) is interpreted to represent the obduction of the Ballantrae Ophiolite and the onset of the Grampian phase of the Caledonian Orogeny in Scotland. Thermodynamic modelling indicates peak conditions of c . 825°C and at least 10 kbar for the unit. In conjunction with a biostratigraphic constraint on metamorphic garnet deposition in sediments at 465 ± 2.5 Ma, this new age more precisely constrains the duration of Grampian metamorphism in Scotland to 12.6 ± 3.1 myr (2σ). In combination with this new age, previous geochronological work on sillimanite-zone garnet indicates a time-averaged initial heating rate of 106 ± 78°C myr −1 (2σ), a rapid rate that may be inconsistent with orogenic self-heating via crustal overthickening alone. The calculated heating rate for the associated garnet zone is also rapid and more precisely constrained at 47 ± 14°C myr −1 (2σ). These results together with the short overall duration of the event support the idea that the Grampian phase of the Caledonian Orogeny in Scotland included both rapid heating and correspondingly fast exhumation.