THE LOWER COVERDALE PLUTONIC SUITE: CA. 975 MA FE-TI-P FERRONORITE AND ALKALI ANORTHOSITE IN THE GANDERIAN BROOKVILLE TERRANE, SOUTHERN NEW BRUNSWICK, CANADA

The Lower Coverdale plutonic suite underlies an area of about 35 km² south of Moncton, New Brunswick. 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. The pluton lies geographically in the Ganderian Brookville terrane, but no host rocks are present in the drill core, which consists entirely of igneous and meta-igneous rocks. The dominant rock types are anorthosite, mafic anorthosite, ferronorite and interlayered Fe-Ti-P rocks. A ferronorite sample yielded an Early Neoproterozoic age of 975.5+7.8/-6.9 Ma, interpreted to be the igneous crystallization age of the plutonic suite.

The anorthosite has characteristics of alkali anorthosite massifs, including andesine plagioclase (An_30-35) with K-feldspar exsolution lamellae containing up to 1.5% BaO. Mafic minerals, originally orthopyroxene (~En₆₀), are mostly altered to amphibole and chlorite. Ferronorite consists of plagioclase and orthopyroxene with varying amounts of apatite and ilmenite, and grades into Fe-Ti-P rocks (nelsonite) with up to 30% TiO₂, 15% P₂O₅, and 1500 ppm V. Anorthosite contains 50-60% silica, with high Na and Ba, whereas ferronorite has SiO₂ ranging from 45-25%, with 25-14% Fe₂O₃, 14-3% TiO₂, and up to 8% P₂O_5. Anorthosite and ferronorite are typically interlayered with sharp contacts that locally suggest that ferronorite is chilled against anorthosite. Ferronorite contains magnesian (En_62-70) orthopyroxene and apatite with 3-4% F. Abundant mafic dikes with high-aluminum tholeiite compositions and minor high-K granitic dykes in both anorthosite and ferronorite are interpreted to be approximately coeval with the anorthosite-ferronorite. Quartz monzonite intersected at the bottom of one hole is Devonian (ca. 390 Ma) and genetically unrelated to the anorthosite suite.

Lower Coverdale appears to be a slightly younger example of > 1 Ga alkalic anorthosite massifs like Labrieville and others in Quebec and Montpelier and Roseland in Virginia. It could be a transported fragment derived from autochthonous Laurentia, or it may have formed in a Mesoproterozoic orogenic belt in Amazonia, to be later separated as part of Ganderia by continental rifting in the break-up of Rodinia.