SIGNIFICANCE OF DETRITAL ZIRCON AGES FROM THE WESTBORO QUARTZITE, AVALON TERRANE, EASTERN MASSACHUSETTS

Quartzites are found in much of the Avalon terrane (sensu stricto) in SE New England (SENE), where they form scattered exposures within intrusive and volcanic rocks associated with the ~ 600 Ma Avalonian arc magmatism. These quartzites, assigned to the Westboro Fm. in MA, the Plainfield Fm. in RI and CT and the Blackstone Group in RI, have been interpreted to pre-date the Avalonian arc magmatism based largely on the assumed age of cross-cutting granites supported by previous studies of detrital zircons where the youngest grain found was ~ 1Ga. We analyzed 95 detrital zircon grains from an orthoquartzite from the Westboro Fm. type locality, Westborough, MA by U-Pb (LA-ICP-MS), but 15 were rejected due to their high discordance. Thirteen zircons lie in the 590-650 Ma age range. The age distribution from the remaining analyses includes age peaks at ~1200, 1500 and 1900 Ma; the oldest zircon is 2688 Ma. Based on the pooled concordia age of the six youngest (concordant and overlapping) analyses, the maximum sedimentation age deduced from this sample is 600±3 Ma (Ediacaran).

Lack of ~ 600 Ma detrital zircons in previous studies of quartzites from SENE (except for one grain in the Plainfield Quartzite in CT; Karabinos and Gromet, 1993) indicates either that this population of zircons was missed due to sample bias or, more likely, that quartzites of more than one age are present in this terrane. We think that at least the belt of quartzite along the western edge of the Avalon terrane from Westborough southward into the Plainfield Fm. of W. RI and E. CT is part of a younger succession that formed post 600 Ma, whereas field evidence suggests the Blackstone Group likely predates the Avalonian magmatism. Quartzites mapped as the Westboro in other areas of E. MA may fall into either group. The younger quartzites must lie unconformably on the ~ 600 Ma Avalonian magmatic rocks and may be as young as Cambrian, when the Boston area was in a stable shelf environment.

The Mesoproterozoic and Paleoproterozoic age populations are consistent with previous detrital zircon studies in SENE and elsewhere in the Avalon composite terrane of the N. Appalachians, strengthening ties of the Boston area to other Avalonian fragments. They also support a non-West African craton connection for western Avalonia and are entirely consistent with an Amazonian craton derivation.