U-PB GEOCHRONOLOGY OF THE ROXBURY CONGLOMERATE, GREATER BOSTON, MA: IMPLICATIONS FOR AVALONIAN TECTONICS IN SE NEW ENGLAND

TIMS U-Pb zircon geochronology constrains Avalonian arc-related magmatic rocks across SE New England between about 610 Ma and 590 Ma, but so far has provided only a ca. 595 Ma maximum depositional age for overlying Roxbury Conglomerate in the Boston Basin. This date reflects the youngest of three igneous clasts and a small sampling of detrital zircons in the Roxbury's uppermost Squantum "Tillite" Member. Now, two CA-TIMS crystallization ages from Brighton volcanic interbeds lower in the Roxbury section establish an absolute age that is approximately 10 Ma younger than the youngest Squantum detritus. Dated samples are auto-brecciated dacite from Brookline, MA (67.51 wt% SiO₂) and amygdaloidal andesite from Newton, MA (61.16 wt% SiO₂). LA-ICPMS analyses from the Squantum sandstone and from Roxbury-related sandstones in Franklin Park (Dorchester section of Boston, MA) and the Webster Conservation Area (Chestnut Hill section of Newton) have also been obtained to document full detrital zircon age profiles.

The new age data cast doubt on the traditional stratigraphic cross section of the Boston Basin showing Roxbury Conglomerate thickening northward and inter-fingering with the Cambridge Argillite. First, the Brighton flows in this scheme lie at or above the level of volcanic ash in the argillite that is some 15 Ma younger. Contrasting detrital zircon profiles in Roxbury sandstones are also difficult to explain in terms of the simple facies model. The Webster and Squantum age distributions are dominated by Ediacaran peaks containing respectively 88% and 69% of analyzed zircons (in both cases n=89); 9 of 12 CA-TIMS dates from grains contributing to these peaks are <600 Ma. The Franklin Park profile (n=92) contains ~70% pre-Ediacaran zircons, and 5 of 10 Edicaran zircons from its Ediacaran peak yielded CA-TIMS dates > 600 Ma. Roxbury Conglomerate deposition is better framed in terms of localized depocenters controlled by a succession of Edicaran normal faults and supplied by different source rocks over time.

The cessation of Avalonian arc magmatism in SE New England is usually pegged at ca. 595 Ma (age of the alkalic Dartmouth Pluton, New Bedford, MA), but the Brighton age constraints, in conjunction with calc-alkaline geochemical signatures of these volcanics, imply that the picture may be more complex.