Timing and heat sources for the Barrovian metamorphism, Scotland

Abstract

New SHRIMP U/Pb zircon ages of 472.2±5.8Ma and 471.2±5.9Ma are presented for the age of peak metamorphism of Barrovian migmatites. 40Ar/39Ar ages for white mica from the Barrovian metamorphic series are presented, and are recalculated using recently-proposed revisions to the 40K decay constants to allow more precise and accurate comparison with U/Pb ages. The 40Ar/39Ar ages are found to vary systematically with increasing metamorphic grade, between c. 465Ma for the biotite zone and c. 461Ma for the sillimanite zone.There is no evidence for any significant metamorphic heating during the first 15. Myr of the Grampian Orogeny (before c. 473. Ma) or the final 4. Myr (after c. 465. Ma). The Barrovian metamorphism occurred over a period of ~. 8. Myr within the ~. 27-Myr Grampian Orogeny. The Barrovian metamorphism records punctuated heating, was temporally and spatially associated with large-scale bimodal magmatism, and developed within crust that was not overthickened. The temporally distinct nature of the Barrovian metamorphic episode within the Grampian Orogeny, and its heating pattern and tectonic context, are not consistent with significant heat contribution from thermal equilibration of overthickened crust. Rather, the Barrovian metamorphism records a transient phase of crustal thermal disequilibrium during the Grampian Orogeny.Temporal and spatial association with Grampian bimodal magmatism is consistent with production of the Barrovian metamorphic series within the middle crust as the result of advection of heat from the lower crust and/or mantle. The Barrovian metamorphic series - the classic example of 'orogenic regional metamorphism' - did not form in response to crustal thickening and thermal relaxation, but appears to record large-scale contact metamorphism. © 2013 Elsevier B.V.