DOES WEST AFRICAN CRUST EXTEND INTO SOUTHEASTERN NEW ENGLAND?

Recently, a piece of Paleoproterozoic West African crust was recognized in the COST No. G-1 core in Georges Bank, offshore Massachusetts (MA). It is interpreted as having been transferred from the Anti-Atlas region of northwest Africa to North America during the formation and subsequent breakup of Pangea. The question then arises as to how far the West African fragment might extend into New England. Most of southeastern MA is interpreted as the Avalon terrane, except a small area southeast of the Nauset anomaly, which is generally interpreted as the Meguma terrane. However, a previously reported but overlooked ~584 Ma (zircon U-Pb) granite from a drill core ~7 km southeast of the Nauset anomaly is older than the oldest rocks exposed in the Meguma terrane of Nova Scotia. The area is more likely to be part of the Avalon terrane or of a West African fragment, which both contain Neoproterozoic granitoid rocks. Likewise, Neoproterozoic granitoid rocks in southeastern MA that have previously been used as evidence for the Avalon terrane may alternatively be part of a West African fragment.

Late Neoproterozoic to Ordovician metasedimentary rocks in southern Rhode Island (RI) are generally also interpreted as part of the Avalon terrane, although some workers have previously considered them to be non-Avalonian. Based on rock types, ages and trilobite occurrences, they may have a West African origin. Based on granitoid ages, zircon inheritance and rock types, it is also possible that rocks interpreted as Avalonian, immediately northwest of the Narraganset Basin of southeastern MA and RI, and those of southernmost RI and Connecticut (CT) are also West African.

Existing U-Pb detrital zircon dates from the Squantum tillite in Quincy, MA, and from the Westboro quartzite in Saugus and Westborough, MA, include some that are within the ~1.7-1.0 Ga age gap of West Africa, and thus interpreted as part of the Avalon terrane. Inherited zircon U-Pb and whole rock Nd model ages of ~1.7-1.0 Ga from igneous rocks ~20 km north and ~5 km west of the Narragansett Basin of MA and RI are also indicative of the Avalon terrane. Thus, while evidence exists for the Avalon terrane in eastern and northeastern MA, parts of southeastern MA, and southern RI and CT may instead be part of a West African fragment. If true, the Pangean suture zone lies on land.