A LOWER CRUSTAL PERSPECTIVE ON THE SCOTTISH AND IRISH CALEDONIDES (Invited Presentation)

Within the Irish and British sectors of the Caledonides, the transition to cratonisation of the orogen is poorly understood, especially the timing and effects of late Caledonian (Acadian) deformation and subsequent thermal events in Ireland. This contribution will address these issues, drawing particularly on evidence from deep crustal xenoliths from Scotland (Midland Valley and Southern Uplands terranes) and Ireland (Leinster-Lakesman terrane).

High grade metamorphic xenoliths from the Scottish Midland Valley and Southern Uplands terranes comprise mainly igneous protoliths (metadiorites and metatonalites) with lesser amounts of metasediments. U-Pb zircon dating shows that Newer Granite-related metadiorite xenoliths formed at 415 Ma. Arc-related metatonalites have protolith ages between 450 – 430 Ma. Detrital zircons from the metasediments imply deposition in late Ordovician to Silurian times and have an age spectrum consistent with a Laurentian provenance, with the addition of a Middle Ordovician component from the Midland Valley arc.

U-Pb dating of zircon metamorphic overgrowths in all three types of xenolith indicates two metamorphic episodes at c. 397 Ma and c. 389 Ma, both possibly linked to the Acadian and potentially correlative with upper crustal deformation elsewhere. An older phase of metamorphism at c. 415 Ma was probably triggered by the combined effects of heating due to the emplacement of the Newer Granite plutons and overthrusting of the Southern Uplands terrane onto the Midland Valley terrane.

Lower crustal xenoliths from central Ireland, derived from the footwall of the Iapetus Suture Zone, are predominantly granulite-facies metasediments. U-Pb dating of detrital zircons and inherited zircons in granitic veins indicate maximum protolith ages of 460 – 476 Ma and an age spectrum consistent with a peri-Gondwanaland (s.l.) affinity. Following ductile deformation, the xenoliths record a series of melting/ high grade metamorphic events between c. 414 Ma - 360 Ma. While some of this complex history correlates with Acadian deformation, much heating continued after it. This thermal anomaly had a profound long-term effect culminating in a significant control on later mineralization.