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.

ABSTRACT Voluminous bimodal volcanic rocks of the Silurian (ca. 422–420 Ma) Dickie Cove Group in the Ganderia domain of northern New Brunswick, Canada, are subaerial units that were deposited in an extensional setting, with the mafic types corresponding to continental tholeiites. Felsic rocks are rhyolites with calc-alkaline affinities. They exhibit geochemical characteristics that are typical of A2-type felsic magmas, such as enrichments in the incompatible elements Zr, Nb, and Y, as well as high FeO*/(FeO* + MgO) and Ga/Al ratios. Their ε Nd ( t ) values are positive (+0.7 to +3.4) but lower than those of the associated basalts. Saturation thermometry has yielded average zircon crystallization temperature estimates for the rhyolites that are well above 900 °C. The geochemical data indicate that the felsic melts were likely sourced from heterogeneous Neoproterozoic lower crust and generated by dehydration melting triggered by heat derived from underplated mafic magma. Parent melts of the rhyolites underwent fractional crystallization in a complex magma chamber prior to eruption. The Nd isotopic data suggest that the lower crust of Ganderia is similar to that of Avalonia in northern mainland Nova Scotia, and that the two microcontinents shared a common Neoproterozoic history and origin as continental blocks rifted from neighboring parts of Gondwana. The tectono-magmatic setting of the Dickie Cove Group volcanic rocks is interpreted as being related to Pridolian, post-Salinic relaxation and slab breakoff, which generated volcanism initially constrained within the Chaleur zone of the Chaleur Bay synclinorium, a large domain of the northern Appalachians. This was followed later in the Pridolian by extensional collapse and widening of the area of magmatic activity, which then prograded into the Tobique zone farther to the southwest.