Geochemical distinctions of late Proterozoic and Paleozoic volcanism in the Avalon zone of New England

Geochemistry permits distinction of two broad groups of volcanic rocks distinguished from the Avalon zone of southeastern New England. An alkalic suite is relatively enriched in K, Rb, Y, Zr, Nb, Zn, La, Ce, but lower in Al, Sr, and Ba compared to a calc-alkaline suite. The alkalic suite is compositionally similar to plutonic rocks that range in age from Late Ordovician to Devonian, whereas the calc-alkaline suite is similar to late Proterozoic plutonic rocks. Geochemical discriminant criteria developed in this chapter indicate that a number of the volcanic units previously have been incorrectly grouped and misinterpreted. The late Proterozoic igneous rocks are compositionally compatible with rocks formed by arc-building processes, and may represent outboard terranes of Africa formed during Pan-African orogenic events. In contrast, the alkalic igneous rocks probably reflect anorogenic or extensional regimes, following earlier crustal thickening processes and removal of first melts from such thickened crust; these rocks may reflect subsequent foundering and break-up of outboard Pan-African terranes that occurred episodically throughout much of the Paleozoic. The presence of alkalic bimodal volcanic rocks within the Narragansett Basin suggests that alkalic igneous activity in southeastern New England was more prolonged than previously recognized, and intermittently continued beyond the Devonian into Carboniferous time, perhaps accompanying the early stages of basin formation. These bimodal volcanics, the only known example of Carboniferous volcanism in the Appalachians of the U.S., are reminiscent of Late Devonian to Early Carboniferous volcanism that preceeded and accompanied the early stages of the Magdalen Basin in the Canadian maritimes.