Orogenesis and basin development: U-Pb detrital zircon age constraints on evolution of the Late Paleozoic St. Marys Basin, central mainland Nova Scotia

The St. Marys Basin, along the southern flank of the composite Late Paleozo ic Magdalen Basin in the Canadian Appalachians and along the Avalon-Meguma terrane boundary, contains Late Devonian-Early Carboniferous continental el astic rocks of the Horton Group that were deposited in fluvial and lacustri ne environments after the peak of the Acadian orogeny. SHRIMP II (Geologica l Survey of Canada) data on approximately 100 detrital zircons from three s amples of Horton Group rocks from the St. Marys Basin show that most of the zircons have been involved in a multistage history, recycled from elastic rocks in the adjacent Meguma and Avalonian terranes. Although there is a mi nor contribution from Early Silurian (411 Ma) and Late Devonian suites (ca. 380-370 Ma), Neoproterozoic (ca. 700-550 Ma) and Paleoproterozoic (ca. 2.0 -2.2 Ga) zircon populations predominate with a minor contribution from ca. 1.0-, 1.2-, and 1.8-Ga zircons. Published U-Pb single-zircon analyses on el astic sedimentary rocks indicate that the Meguma and Avalon terranes have d ifferent populations of detrital zircons, sourced from discrete portions (A mazonian and West African cratons) of the ancient Gondwanan margin. Both te rranes contain Neoproterozoic and Late Archean populations. The SHRIMP data , in conjunction with published sedimentological and geochemical data, indi cate that the Horton Group basin-fill sediments are largely the result of r apid uplift and erosion of Meguma terrane metasedimentary and granitoid roc ks immediately to the south of the St. Marys Basin during the waning stages of the Acadian orogeny. Regional syntheses indicate that this uplift occur red before and during deposition and was a consequence of dextral ramping o f the Meguma terrane over the Avalon terrane along the southern flank of th e Magdalen Basin.