EVOLVING VIEWS OF AVALONIA IN SOUTHEASTERN NEW ENGLAND: PERSPECTIVES FROM EDIACARAN CAMBRIDGE "ARGILLITE", EASTERN MASSACHUSETTS

High-precision CA-TIMS ²⁰⁶Pb/²³⁸U zircon dates clarify the age and tectonic significance of the Cambridge Formation which is poorly exposed in the Boston Basin, eastern Massachusetts, but transected by ~ 50 km of tunnels beneath the mainland and Boston Harbor. The youngest detrital zircon in a sample from the northern Braintree Weymouth Tunnel establishes a maximum depositional age of 584.09 ± 1.98 Ma, consistent with sources in sills of that age in underlying Roxbury Conglomerate. A 551.22 ± 0.20 Ma ash bed from the Mystic Quarry in Somerville, MA lies near the top of ~ 5350 m of dominantly argillaceous section measured in subsurface cross sections. These were constructed from attitudes reported in pre-1960s tunnels and from mapping logs in tunnels completed decades later during the federally ordered clean-up of Boston Harbor. A 488.58 ± 0.16 Ma aplite sill intruding argillite ~800 m above the ash bed sets the minimum depositional age on the north side of the Basin. A tighter constraint comes from trilobite-bearing lower Cambrian strata overlying argillite without obvious break south of Boston in the Braintree Weymouth Tunnel. If Cambridge deposition was continuous, the depositional interval would exceed 40 million years. A post-584 Ma depostional hiatus of uncertain duration seems more likely because the base of the Cambridge Fm appears to be a regional unconformity above which argillite rests variously on magmatic arc-related units of both the 595-584 Ma Roxbury Conglomerate and the 597-593 Ma Lynn-Mattapan Volcanic Complex.

Cambridge deposition, in any case, set in once arc activity in more northerly "West" Avalonian terranes extending to the Avalon Peninsula, Newfoundland had given way to wrench faulting and bimodal magmatism. This regime is manifested structurally in Boston-area tunnels by later-reactivated normal faults in which hanging wall blocks of Cambridge argillite were originally downthrown relative to older footwall units. Pyroclastic textures and thin mafic flows with soft sediment contacts are present in northern argillite, and whole rock major element and REE compositions reveal a mixture of terrigenous and volcanic components throughout the Basin. Proposed sources for the latter are voluminous eruptions recorded in the 560-550 Ma Coldbrook Group in New Brunswick's Caledonia terrane.