Sediment provenance and tectonics on the Laurentian margin: implications of detrital zircons ages from the Central Belt of the Southern Uplands-Down-Longford Terrane in Co. Monaghan, Ireland

摘要

The Ordovician-Silurian meta-sedimentary rocks of the Southern Uplands-Down-Longford terrane were deposited at the Laurentian margin and structurally emplaced in an accretionary prism during subduction of Iapetus Ocean lithosphere. Within the terrane, a Central Belt consists of repeated structural tracts in which Ordovician-Silurian mudstone and sandstone (Moffat Shale Group) are overlain by Silurian greywacke (Gala Group). A Moffat Shale Group sandstone and Gala Group greywacke sandstones from Co. Monaghan, Ireland were sampled in order to examine, for the first time, detrital zircon spectra with a view to elucidating sediment provenance and tectonic evolution of this sector of the Laurentian margin. Detrital zircon ages from Gala Group greywacke sandstones include variable abundance of Precambrian zircons with ages typical of Laurentia, but are dominated by two Ordovician peaks at c. 462 Ma and c. 474 Ma. These Ordovician ages equate to magmatism in the local Laurentian margin in Ireland. The age spectra are distinct from those of equivalent rocks of the Southern Uplands of Scotland. There is no evidence from detrital zircon ages that a Gondwana-derived terrane had accreted to the Laurentian margin before or during sedimentation. Zircons from a Moffat Shale Group sandstone include a middle Cambrian component that may be derived from Ganderia.