Lower Paleozoic tholeiitic dykes from central New Brunswick: possible evidence for the early opening of an ensialic Taconian back-arc basin

Abstract

Cambrian and Early Ordovician sedimentary rocks of the Lower Tetagouche Group from the southern Miramichi Terrane in New Brunswick were intruded by a series of mafic dykes. The age of dykes is unknown, but their stratigraphic context and state of deformation indicate that they were intruded during the Taconian orogeny. The geochemistry of the dykes shows that they are a cogenetic magmatic suite of basaltic composition (48–55 wt.% SiO2) and tholeiitic affinity. The dykes are low in silica and high in TiO2 (1.5–3.5 wt.%) and FeO* (8–16 wt.%). Magmatic evolution of the suite resulted from processes involving both crystal fractionation and crustal contamination, as indicated by depletion of compatible elements, important enrichment in high-field-strength elements, and the presence of inherited zircons of Precambrian age. The most primitive samples resemble mid-ocean-ridge basalts, except for higher alkali contents, or continental tholeiites depleted of light rare-earth elements. In comparison, the most evolved dykes have a transitional character similar to within-plate basalts.The general evolution of the Lower Tetagouche Group dykes is similar to that of nascent basins formed within volcanic-arc complexes. Dyke intrusion resulted from the early development of an ensialic back-arc basin during the Taconian orogeny. The paroxysmal stage of this tectonic event is represented by the ocean-floor fragment of the Elmtree Terrane, north of the Miramichi Terrane.