Abstract

Electron-microprobe geochronology on detrital monazite grains from fluvial sandstones of the Lower Cretaceous Chaswood Formation onshore Nova Scotia and New Brunswick and from fluvial and deltaic sandstones of the Missisauga and Logan Canyon formations of the offshore Scotian basin was determined to understand sediment dispersal patterns and paleogeography. After removing grains showing Th or U mobility and single analyses that were extreme outliers, ages from 393 analyses from 78 monazite grains were interpreted. In addition, monazites were classified into six types based on their rare-earth element abundance, determined by electron microprobe analysis, and six types of chemical zoning, interpreted from backscattered electron and wavelength dispersive spectrometer images.

Detrital monazite grains from southern New Brunswick yielded Silurian ages exclusively, whereas in central Nova Scotia, Ordovician ages predominate, with a few grains of Cambrian or latest Neoproterozoic ages in eastern Nova Scotia. Monazite in the Scotian basin is predominantly of Ordovician age, with a significant number of Mesoproterozoic and Paleoproterozoic grains. Monazite grains with well-developed zoning from southern New Brunswick and eastern Nova Scotia have a high-grade metamorphic source; common grains with patchy, low contrast zoning have a source from low-to intermediate-grade metamorphic rocks; and unzoned monazite grains that predominate in central Nova Scotia, with Y-rich rims, may have a granite or migmatite source.

Variations in detrital monazite age and composition suggest that several rivers transported sediment southwards from multiple bedrock sources. One supplied southern New Brunswick, one central Nova Scotia, at least one in eastern Nova Scotia, and one or more rivers draining western Newfoundland were the principal source of monazite to the main depocentre near Sable Island.