PALEOZOIC VOLCANISM IN THE BOSTON BASIN, EASTERN MA: EVIDENCE FROM GEOCHRONOLOGY AND PALEOMAGNETISM

The geologic history of the Boston Basin and surrounding uplands has largely been written in Avalonian terms of Neoproterozoic arc magmatism and ensuing Cambrian platform sedimentation, but mounting evidence reveals Paleozoic volcanism as well. The youngest U-Pb zircon dates obtained from lava flows at Houghs Neck (Quincy), Hewitts Cove (Hingham), Webster Conservation area (Newton) and Nantasket Beach are 587 Ma (Neoproterozoic), 490 Ma (Cambro-Ordovician), 438 Ma (Silurian) and 362 Ma (Late Devonian), respectively. All of the flows also contain older zircons. Because these results must be interpreted as maximum eruptive ages, geochronology cannot yet resolve the number of volcanic episodes. Paleomagnetic results from the same samples, however, fall into two distinct groups. Tilt-corrected results from the Webster Conservation Area and Hewitts Cove both show normal and reversed polarities with inclinations indicative of moderate to low latitudes. These poles resemble published results from the Late Silurian Pembroke Formation, New Brunswick and the Early Devonian Dalhousie Group, Nova Scotia. In contrast, tilt-corrected data from Nantasket volcanic rocks and the Houghs Neck flow and overlying red siltstone yield equatorial paleolatitudes. The Nantasket and Houghs Neck poles agree well with Late Devonian-Carboniferous North American poles from the Catskill redbeds and the Mauch Chunk Formation in the northern Appalachian foreland. The similarity of the Houghs Neck pole with Nantasket can either mean that the Houghs Neck rocks are younger than Neoproterozoic or that they were remagnetized (although the positive conglomerate test obtained here suggests that the magnetization is primary).

Much U-Pb work remains to establish absolute ages for these inheritance-plagued lava flows which are currently designated as Brighton volcanic interbeds in assumed Neoproterozoic Roxbury Conglomerate. Nevertheless, the preliminary single-zircon dates from Nantasket alone leave no doubt that there are Paleozoic volcanic rocks in the Boston Basin. Given that "Brighton" volcanic rocks have previously been linked geochemically with subduction-related settings, the emerging stratigraphy points towards Acadian and/or Neo-Acadian chapters in the tectonic history of the Southeastern New England Avalon Zone.