NEW U-PB DETRITAL ZIRCON AGES FROM METASEDIMENTARY ROCKS ALONG THE WESTERN-MOST BOUNDARY OF THE INTERPRETED AVALON TERRANE IN THE SOUTHEASTERN NEW ENGLAND APPALACHIANS - A POSSIBLE WEST AFRICAN SOURCE AREA?

The Avalon terrane (AT) is a microcontinent that rifted from supercontinent Gondwana in the Ordovician and accreted to Laurentia during the latest Silurian to Devonian Acadian orogeny. Today, the extent of the type AT is well constrained in Canada by U-Pb detrital zircon studies and/or isotope geochemistry of (meta)sedimentary and igneous rocks. In the New England Appalachians, however, the western and southeastern parts of the currently interpreted AT may instead be interpreted as other (micro)continents.

U-Pb detrital zircon analysis of three samples of quartzite +/- interbedded metapelitic rocks along the western boundary of the AT in eastern MA and CT was conducted to test whether they are part of the AT, or have other affinities. Based on ²⁰⁶Pb/²³⁸U weighted mean ages from the youngest distinctive age populations, the Westboro Fm. in northern MA (n = 113) and the Plainfield Fm. along the MA-CT border (n = 76) have maximum depositional ages of ~735 Ma (MSWD = 0.007; n = 2) and ~950 Ma (MSWD = 0.07; n = 3), respectively. The third sample (n = 99), from the Plainfield Fm. in east-central CT, has a maximum depositional age of ~530 Ma (MSWD = 0.31; n = 2). A total of only 8 grains with ~500-700 Ma ages were present in the northern and southern-most samples which is atypical for Cambrian and younger rocks in the type AT. All three samples show large ~1.0 Ga, ~1.2 Ga, and ~1.5 Ga populations, which may be correlated with data from Laurentia, Mauritania, or some other previously unrecognized microcontinent(s). Previous studies within the interpreted AT (e.g., Hepburn et al., 2008, Thompson et al., 2007) have found similar atypical U-Pb detrital zircon results with or without the ~500-700 Ma population.

The occurrence of Laurentian rocks between Avalonia and Ganderia, a separate Gondwanan terrane that lies west of the AT, would need to be explained. However, if the source is Mauritania or equivalent domains, the rocks may be Avalonian, and at least part of Avalonia would have been derived from the West African part of Gondwana.