Lower Coverdale and Gaytons: Middle Devonian and possibly older anorthosite-ferronorite, gabbro, and quartz monzonite intrusions in southeastern New Brunswick

The Lower Coverdale intrusion near Moncton, New Brunswick, has been intersected in drill holes at depths of 100–200 m below unconformably overlying Carboniferous sandstone, onglomerate and, locally, limestone of the Windsor, Mabou, and Cumberland groups. A large positive aeromagnetic anomaly suggests that the intrusion has a subsurface area of at least 30–40 km2. As revealed by drill core and cuttings, the intrusion consists of interlayered coarse-grained anorthosite and ferronorite, both intruded by gabbro, quartz monzonite, and minor felsic dykes. The ferronorite is high in Ti and P, and contains interstitial patite and ilmenite/magnetite and layers of apatite-ilmenite rock (nelsonite) up to several metres thick. Much of the core shows pervasive effects of metamorphism and alteration but microprobe analyses of the freshest samples revealed that the plagioclase in both anorthosite and ferronorite has andesine composition. The anorthosite and ferronorite are chemically distinct, but their close spatial association suggests a genetic link. In contrast, the younger gabbroic rocks differ in mineralogy and chemistry from, and appear unrelated to, the anorthosite and ferronorite. They are altered but not metamorphosed, and preserve intergranular textures. They contain more calcic plagioclase and augite, and have low Ti and P. The deepest drill hole in the Lower Coverdale intrusion encountered highly altered coarse-grained quartz monzonite at a depth of 1095–1206 m. The quartz monzonite is mineralogically and chemically similar to quartz monzonite in quarries near Gaytons, 20 km to the east. Virtually identical Middle Devonian U-Pb zircon ages of 90.6 ± 1.0 Ma and 390.0 ± 0.5 Ma were obtained for samples from the Lower Coverdale and Gaytons quartz monzonite, espectively. However, the anorthosite-ferronoritegabbro is likely considerably older: perhaps ca. 540 Ma like gabbroic rocks elsewhere in the Brookville terrane; or possibly esoproterozoic, like intrusions with similar characteristics in Grenvillian parts of the Precambrian shield. RÉSUMÉ On a croisé l’intrusion de Lower Coverdale près de Moncton (Nouveau-Brunswick) dans des puits forés à des profondeurs de 100 à 200 m au-dessous de grès du Carbonifère sus-jacent non concordant, de conglomérat et, par endroits, de calcaire des groupes de Windsor, de Mabou et de Cumberland. Une anomalie aéromagnétique positive étendue permet de supposer que l’intrusion a une superficie souterraine d’au moins 30 à 40 kilomètre