Age and Stratigraphic Affinity of Neoproterozoic Conglomerate at Neponset River Gorge, Milton, MA: Constraints from U-Pb Geochronology

Conglomerate in cliffs along the Neponset River in Milton, Massachusetts is largely composed of volcanic fragments as clasts and as sand-sized matrix grains, both with compositions ranging from basalt to rhyolite (including flow banded and pyroclastic varieties). Likely sources for this detritus are diverse Mattapan volcanic rocks underlying the conglomerate in Milton and upstream at the Mattapan type locality. Neponset conglomerate can be traced northward in the Dorchester Tunnel where gradational contacts with felsic volcanic rocks are reported.

A Neoproterozoic age for the entire volcano-sedimentary assemblage is constrained by U-Pb geochronology on a flow-banded rhyolite from Milton. Single zircons, pre-treated by the thermal annealing and acid leaching method and analyzed by the isotope dilution technique (CA-TIMS), produced statistically coherent results with a weighted mean ²⁰⁶Pb/²³⁸U date of 593.49 ± 0.30 Ma (MSWD = 0.87) based on four of five analyses. This date is interpreted as the crystallization age of the rhyolite, and it compares closely to our ca. 597-596 Ma dates from Mattapan volcanic rocks elsewhere. A fifth concordant zircon with a ²⁰⁶Pb/²³⁸U date of ca. 608 Ma represents a xenocrystic component possibly contributed by older Dedham Granite to ensuing rhyolitic volcanism. A sample of Dedham Granite beneath Mattapan ash-flow tuff west of the field area in Hyde Park has produced a weighted mean ²⁰⁶Pb/²³⁸U date of 609.00 ± 0.14 Ma (MSWD = 0.9) based on seven of nine analyzed single zircon grains.

The volcanic-dominated conglomerate described here falls within the Hyde Park-Mattapan belt of Crosby (1880). This belt continues eastward in Hingham and Hull where we previously documented volcaniclastic sediments interbedded with ca. 597 Ma Mattapan volcanic ash resting unconformably on ca. 608 Ma Dedham Granite. Crosby's Brookline-Roxbury belt in the central part of the Boston Basin is distinguished by conglomerate containing quartzite and granite along with volcanic clasts.