ABSTRACT Granitoid batholiths dominated by felsic to intermediate compositions are commonly associated with mafic plutons and enclaves; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is unclear. Here, we reviewed the age and lithogeochemical and Nd-Sr isotopic compositions of some classic plutonic rocks emplaced in the Northern Highlands, Grampian and Connemara terranes of the Caledonide orogen of Scotland and Ireland. The Northern Highlands terrane consists mostly of Neoproterozoic metasedimentary rocks of the Moine Supergroup and is located north of the Great Glen fault. The Grampian terrane also consists of Neoproterozoic metasedimentary rocks (Dalradian Supergroup) and is located south of the Great Glen fault in both Scotland and Ireland. Amphibolite-facies metasedimentary rocks in the Connemara terrane are correlated with the Dalradian Supergroup, and the terrane is bounded by splays of the Highland Boundary and Southern Uplands faults. These three terranes were intruded by Silurian–Devonian mafic and felsic to intermediate plutonic rocks that display field evidence for mingling and mixing and have a similar range (between ca. 437 and 370 Ma) in emplacement ages. This range implies they were intruded during and after the late Caledonian Scandian orogenic event that resulted from the mid- to late Silurian collision of amalgamated Avalonia and Baltica with Laurentia and the final closure of the Iapetus Ocean. Our review supports the contention that the Great Glen fault represents a major compositional boundary in the Silurian lithosphere. Felsic to intermediate plutons that occur north of the Great Glen fault are more enriched in light rare earth elements and Ba-Sr-K compared to those to the south. Isotopic compositions of these late Caledonian plutonic rocks on both sides of the Great Glen fault indicate that metasomatism and enrichment of the subcontinental lithospheric mantle beneath the Northern Highlands terrane occurred just prior to emplacement of late Caledonian plutons. Within the same terrane, mafic and felsic to intermediate rocks display similar trace-element and rare earth element concentrations compatible with models implying that fractionation of a mafic magma played an important role in generating the felsic to intermediate magmas. The onset of slab failure magmatism may have been diachronous along the length of the collision zone. If so, slab failure may have propagated laterally, possibly initiating where promontories collided.

ABSTRACT Three Silurian basin fills, the Llandovery–Wenlock Croagh Patrick and Killary Harbour–Joyce Country successions and the Ludlow–Pridoli Louisburgh–Clare Island succession, overstep the tectonic contacts between elements of the Grampian (Taconic) accretionary history of the Caledonian-Appalachian orogeny in western Ireland. New U-Pb detrital zircon data from lower strata of these Silurian rocks provide insight into basin evolution and paleogeography. The shallow-marine Croagh Patrick succession unconformably overlies the Clew Bay Complex and the northern part of the Ordovician South Mayo Trough. Two samples have zircon populations dominated by Proterozoic grains typical of the Laurentian margin, with few younger grains. Up to 13% of the grains form a cluster at ca. 950–800 Ma, which is younger than known Grenville magmatism on the local Laurentian margin and older than known magmatism from Iapetan rifting; these may be recycled grains from Dalradian strata, derived from distal Tonian intrusions. The Killary Harbour–Joyce Country succession overlies the structural contact between the Lough Nafooey arc and the Connemara Dalradian block and records a transgressive-regressive cycle. Four samples of the Lough Mask Formation show contrasting age spectra. Two samples from east of the Maam Valley fault zone, one each from above Dalradian and Nafooey arc basement, are dominated by Proterozoic grains with ages typical of a Laurentian or Dalradian source, likely in north Mayo. One sample also includes 8% Silurian grains. Two samples from west of the fault overlie Dalradian basement and are dominated by Ordovician grains. Circa 450 Ma ages are younger than any preserved Ordovician rocks in the region and are inferred to represent poorly preserved arc fragments that are exposed in northeastern North America. Cambrian to late Neoproterozoic grains in association with young Ordovician ages suggest derivation from a peri-Gondwanan source in the late stages of Iapetus closure. The Louisburgh–Clare Island succession comprises terrestrial red beds. It unconformably overlies the Clew Bay Complex on Clare Island and is faulted against the Croagh Patrick succession on the mainland. The Strake Banded Formation yielded an age spectrum dominated by Proterozoic Laurentian as well as Ordovician–Silurian ages. Although the basin formed during strike-slip deformation along the Laurentian margin in Ireland and Scotland, sediment provenance is consistent with local Dalradian sources and contemporaneous volcanism. Our results support ideas that Ganderian continental fragments became part of Laurentia prior to the full closure of the Iapetus Ocean.