Late Ordovician to Early Devonian tectono-magmatic prequel to the Acadian Orogeny in northeastern North America and the British Isles

Abstract

Geochemical data from Katian to earliest Emsian (∼453–405 Ma) igneous rocks in northeastern North America and the British Isles were compiled to identify tectono-magmatic events related to ocean closure and the formation of the Appalachian–Caledonian Belt. These rocks all have geochemical affinities with plate-margin settings, but only a few can be attributed to arc magmatism, whereas the others have slab-failure signatures or affinities with anhydrous, extensional plate-margin (A2-type) settings. Based on these setting attributions as well as constraints from the palaeomagnetic, palaeontologic, structural, stratigraphic and sedimentologic records, a model for Iapetus and Rheic ocean closure is proposed, which also involves three subordinate ocean plate segments: the Tornquist Sea, Acadian Seaway and Tetagouche–Exploits oceanic back-arc basin. The model includes several new perspectives, such as (1) an early Silurian rather than late Silurian closure of the Tetagouche–Exploits back-arc basin; (2) Acadian Seaway slab failure at the Ludlow–Pridoli boundary due to its interaction at depth with the overlying and slowly-sinking Tetagouche–Exploits slab, which generated profuse, extensional, A2-type volcanism; and (3) an Early Devonian reactivation of Acadian Seaway slab subduction, possibly due to Rheic Ocean closure and the convergence of a Gondwanan promontory against Avalonia, which was attached to oceanic lithosphere of the Acadian Seaway. Furthermore, age constraints allowed to identify chronological trends in the geochemical signatures of the igneous rocks under study, which suggest that development of a new tectono-magmatic signature was gradual due to compositional inheritance from the previous setting. These trends also suggest that, although the transition from active subduction to slab failure generates an increase in Nb/Y and light over heavy rare earth elements, these ratios tend to decrease with time due to a fading contribution of the sinking slab at the source, whereas high-field-strength element contents tend to increase due to a lack of new water input from subduction.