PALEOMAGNETIC/GEOCHRONOLOGIC STUDY OF THE BOSTON BASIN AND ENVIRONS, EASTERN MASSACHUSETTS

Preliminary results are now emerging from integrated paleomagnetic and geochronologic work in the Boston Basin portion of the Southeastern New England Avalon Zone. Approximately 250 oriented cores have been analyzed from 15 locations both in and around the basin. Most samples are from Neoproterozoic formations including the Dedham Granite (ca. 610 Ma), the Westwood Granite (ca. 599 Ma) and Brighton volcanic intebeds of the Roxbury Conglomerate (maximum age 587 Ma at Houghs Neck, Quincy). We also sampled post-Avalonian magmatic rocks including the alkalic Quincy Granite (shown as Ordovician-Silurian on the Bedrock Geologic Map of Massachusetts) and associated Hancock Porphyry and Wampatuck Volcanics in the Blue Hills (U-Pb zircon dating in progress).

Samples from most locations yield stable directions after demagnetization. Some locations show single component magnetizations, while others show multi-component magnetizations. Structural corrections have been made for almost all of the locations, and fold tests and conglomerate tests have been done wherever possible. We will discuss the likelihood of remagnetization in the various units.

Most of the lithologies sampled produced moderate (between 30° and 60°) inclinations indicative of mid-latitudes. Results from Neoproterozoic samples differ significantly from previously published results for Brighton volcanic rocks and Roxbury siltstones. Especially intriguing are similarities between the Lower Paleozoic Blue Hills suite and flows and tuffs at Hull, Massachusetts that are traditionally mapped as Brighton Volcanics. Ongoing geochronology will clarify whether these similarities reflect Paleozoic remagnetization of Neoproterozoic flows or whether previously undetected, younger volcanic units are also present in the Boston Basin. Paleomagnetic results from dated rocks will be compared with results from other Avalonian terranes and from North America.