Slab-failure or Slab-success? Examining the contributions of crust and mantle to post-subduction magmatism in the Ratagain Complex, NW Scotland.

The Ratagain Complex is an enigmatic Late Caledonian granitic intrusion and a member of the high BaSr Northern Highlands granite (NHG) suite that has been related to slab failure. Slab failure magmatism explains varying contributions of mafic and felsic magmas in post-collision orogenic settings. It is therefore of major importance in understanding crustal accretion. However, the source and nature of any mantle derived contribution is poorly understood. This study reveals that Ratagain is not only transitional in nature between the high BaSr calc-alkaline granites and syenite intrusions of the Northern Highlands Terrane, but overlaps with the entire compositional range of the NHG suite. New lithogeochemical data from Ratagain confirm remarkably high Sr (>1600 ppm) and Ba (>2200 ppm) contents, high LREEs, notable depletions in Nb, U, P and Ti, low HREEs and negligible Eu anomalies, associated with high initial 87Sr/86Sr (0.7055 to 0.7062) and low εNd (− 11.8 to −13.3). Although mafic parts of the complex have strong elemental and isotopic similarities with broadly coeval lamprophyres, signalling derivation from enriched mantle sources, details of the isotope array with respect to local crustal reservoirs indicate a significant Lewisian component. Such geochemical characteristics, combined with tectonic and petrological evidence, may be attributable to long-lived, incremental emplacement of successive magma batches originating from the same enriched mantle but differing in age and extent of assimilation-fractionation crystallisation. We therefore propose that some of the age dates for the Late Caledonian intrusions, particularly those obtained from older geochronology studies, are in need of review as they may record early crystallisation in the deep crust and not be a valid proxy for granite emplacement.