BASALTIC LAVA FLOWS AT WORLDS END RESERVATION, HINGHAM, MASSACHUSETTS: NEOPROTEROZOIC(?) VOLCANISM IN THE SOUTHEASTERN NEW ENGLAND AVALON ZONE

Mafic volcanic rocks associated with Roxbury Conglomerate in the Boston Basin, eastern Massachusetts, have long been inferred to reflect Neoproterozoic arc magmatism. However, preliminary U-Pb geochronology of these “Brighton” lava flows in several localities throughout the region has yielded zircons ranging from Neoproterozoic to Devonian in age. Inheritance is a problem in all of these rocks, so results to date establish only maximum ages. New work on flows and associated volcanic ash cropping out south of Boston at Worlds End Reservation, Hingham, Massachusetts presents the opportunity to better constrain the age of the volcanism.

Seacliff exposures at Worlds End show a series of four conglomerates and two interbedded “Brighton” flows, all uncomformably overlying the Dedham Granite. The upper flow is conspicuously vesiculated and contains albitized plagioclase laths in a matrix of albite, epidote, actinolite, chlorite, and quartz with accessory titanite, iron oxide and copper sulfide. Also present are iron oxide-rimmed clots of epidote that may be pseudomorphs of mafic phenocrysts. The flow is high-alumina basalt with 46.2% SiO₂ and 19.03% Al₂O₃ by weight, but more detailed geochemical characterization is difficult because of deuteric and/or secondary alteration. The basalt is sub-alkalic based on its Nb/Y ratio of 0.19, and it falls in the tholeiitic field on a plot of P₂O₅ vs. TiO₂. However, it appears in the alkalic field when plotting V vs. Ti or P₂O₅ vs. Zr. High-alumina basalts are also reported among "Brighton" flows elsewhere in the Boston Basin at Newton and Nantasket, MA. On a Ti-Zr-Y plot, all of these rocks straddle the boundary between fields for within-plate and calc-alkali basalts, so that the tectonic setting remains ambiguous. Determining a precise age for this volcanism may also help to resolve this question.

The best target for U-Pb geochronology at Worlds End is yellowish gray volcanic ash which fills vesicles at the top of the basalt and also forms a layer overlying the flow. The ash contains 66.98% SiO₂ consistent with a dacitic composition, but appears as a trachyandesite in a Winchester plot of Zr/TiO₂ vs. Nb/Y. The ash is composed of intergrown epidote, albite, ferroactinolite, calcite, and quartz, with accessory titanite, chromite, iron oxide, copper sulfide and zircon which will be dated.