AVALONIAN ARC-TO-PLATFORM TRANSITION IN SOUTHEASTERN NEW ENGLAND, USA: NEW U-PB ZIRCON AGE CONSTRAINTS, MORE QUESTIONS

Avalonian arc magmatism across SE New England produced widespread 609 to 589 Ma granitoid plutons and 597 to 593 Ma calc-alkaline volcanic rocks overlain in the Boston Basin by Roxbury Conglomerate intruded by the 585-584 Ma Brighton Igneous Suite. Thousands of meters of gray siltstone, mudstone and fine sandstone ("argillite") comprising the Cambridge Formation at the top of the Boston Basin section record the transition beginning after 580 Ma to stable Cambrian platform conditions according to the 2006 Appalachian lithotectonic map. New U-Pb single-crystal zircon geochronology carried out on a volcanic ash bed from argillite in Somerville, MA using a combination of LA-ICPMS and CA-TIMS methods now establishes a maximum depositional age in the northern part of the basin that is 20 m.y. younger than the ²⁰⁷Pb/²⁰⁶Pb date of ~570 Ma previously reported for the same sample. A latest Cambrian minimum depositional age has also been obtained from aplite cutting argillite in a core boring several km to the south in Cambridge, MA.

Cambridge strata underlying the southern half of the Boston Basin remain poorly constrained as < ~599 Ma, the youngest of four ²⁰⁷Pb/²⁰⁶Pb detrital zircon dates from siltstone sampled in the Braintree-Weymouth Tunnel. The southern argillites are arguably older than those in the north because locally they contain ring fossils resembling Aspidella that ranges between ~565 and ~557 Ma in the Charnian Supergroup of southern Britain.

The Cambridge Formation has customarily been correlated with Newfoundland's Conception Group, but ash beds in the Drook and Mistaken Point formations have lately yielded respective CA-TIMS dates of ~571 and ~566 Ma. The younger Cambridge dates suggest more likely correlatives in the overlying St. John's Group, especially the ~1400 m Fermeuse Formation with its exceptionally abundant Aspidella fauna. The available geochronology implies a considerable gap between arc magmatism in SE New England and strata transitional to Cambrian platform deposits. Identifying possible missing links in the Boston-area stratigraphic sequence and developing a basin-wide structural interpretation will require synthesis of subsurface records and samples from sewerage tunnels beneath Boston Harbor. And more U-Pb geochronology!