Tonian Fe-Ti-P ferronorite and alkali anorthosite in the northern Appalachian orogen, southern New Brunswick, Canada: Amazonian basement in Ganderia?

Abstract

The Lower Coverdale plutonic suite is located in the Ganderian Brookville terrane of southern New Brunswick, Canada. It does not outcrop at surface but has been intersected in 11 drill holes at depths of 100–200 m below unconformably overlying Carboniferous sedimentary rocks, and extends to a depth of at least 1100 m. No host rocks are present in the drill core, which consists entirely of plutonic and meta-plutonic rocks. The dominant rock types are anorthosite, mela-anorthosite, and ferronorite with layers of ilmenite and ilmenite-apatite rocks. They are intruded by abundant undated mafic dykes and minor ca. 390 Ma felsic dykes. A ferronorite sample yielded a Tonian age of 975.5 ± 2.3 Ma, interpreted to be the igneous crystallization age of the plutonic suite. A mela-anorthosite sample yielded a similar but less precise age of 979 ± 16 Ma, likely also the igneous crystallization age. The anorthosite has characteristics of alkali anorthosite massifs, including andesine (An30-35) commonly with exsolution lamellae of K-feldspar. The Lower Coverdale suite is interpreted to have been emplaced during orogen-scale extension following crustal thickening associated with the Grenvillian orogeny. The Lower Coverdale anorthosite and ferronorite have more isotopically evolved signatures (higher 206Pb/204Pb and 207Pb/204Pb and lower ∊Nd) compared to typical Laurentian Grenville-age plutons. We infer from these data that the Lower Coverdale plutonic suite originated in the Amazonian part of the Grenville orogen during the early stages of Tonian breakup of Rodinia. It was then incorporated into Ganderia before accreting to Laurentia as part of the Brookville terrane during the Pangean phase of supercontinent assembly.