Relationships of coticule geochemistry to stratigraphy in the Perry Mountain and Megunticook Formations, New England Appalachians

Coticules (garnet-rich quartzites) are chemically distinctive lithologies of controversial origin. They generally occur in pelitic schists, amphibolites, cherts, and quartzites as thin layers or discontinuous lenses in Paleozoic formations in the New England Appalachians and other orogenic belts. Geochemical data were collected to characterize coticules and their host rocks from the Middle Silurian Perry Mountain Formation of southern New Hampshire (near Rochester) and western Maine (near Rangeley), and the Cambrian Megunticook Formation of coastal Maine (Camden and Calais). Relative to their host rocks, the coticules are enriched in Fe, Mn, and P, and depleted in Ti and alkalis. Electron-microprobe analyses of garnet show higher spessartine and lower almandine components relative to garnet in the host. Coticules are light-REE-enriched and heavy-REE-depleted. They have concentrations between 10 and 100 times those of chondrites, generally lack a Cc anomaly, and have a negative Eu anomaly. Major-element compositions and REE profiles argue strongly against a hydrothermal origin for these rocks, and do not obviously support a hydrogenous (diagenetic) origin such as Mn-Fe formation. Rare-earth-element patterns differ as much within a local area as for widely separated samples within a single formation. All three samples from the Megunticook Formation are nearly identical, supporting stratigraphic correlation. But they are also like some of the Perry Mountain Formation samples, so that coticule compositions for a formation are not unique.