Collaborative Research: Stratigraphic Resolution as Key to Tectonic Synthesis in the Southeastern New England Avalon Zone Around Boston, Mass.

Ohio State University Research Foundation -Do Not Use, Columbus OH

Investigators

Abstract

Integrated paleomagnetic/geochronologic study necessitates radical tectonic re-thinking in the Boston portion of the Southeastern New England Avalon Zone because new isotopic data reveal fundamental flaws in the long accepted regional stratigraphy. For example: 1. Reportedly Neoproterozoic, but previously undated Brighton volcanic interbeds from four geographically separated sections in the Roxbury Conglomerate have maximum ages of approximately 587 Ma, 490 Ma, 438 Ma and 362 Ma based on 1-4 grain zircon fractions. 2. Reportedly Ordovician Quincy Granite (based on an upper intercept date of 437 + 23 Ma) has yielded Siluro-Devonian zircons (single grain analyses with 207 Pb/ 206 Pb dates of approximately 410 Ma). 3. Reportedly 630 + 15 Ma Dedham Granite west of the Boston Basin has yielded a weighted mean 207 Pb/ 206 Pb date of approximately 610 Ma based on three single zircon analyses. Preliminary paleopoles based on these geochronological data further suggest that: 1. The previously reported Neoproterozoic Avalonian pole form the Roxbury Conglomerate is from Silurian or younger rocks. 2. Reliably dated Neoproterozoic rocks formed at low-mid latitudes rather than the previously reported high-mid latitude for the 610 to ~ 570 Ma interval. This proposal seeks funding for continued work to clarify stratigraphic and structural relationships in the Boston area as a basis for completing 5 or 6 well dated paleomagnetic poles. These poles will constrain revised interpretations of the paleolatitude of the Southeastern New England Avalon Zone during Neoproterozoic arc magmatism, its position during during hypothesized "snowball" earth climatic events and the timing of mid-Paleozoic accretion to North America. Oriented paleomagnetic cores will be collected at 21 new locations in Avalonian arc rocks including Dedham Granite and Mattapan Volcanic Complex, in overlying Neoproterozoic strata including the Cambridge Argillite and glaciomarine deposits comprising the Squantum Member of the Roxbury Conglomerate, in "Brighton" Volcanics and in several other formations of uncertain or poorly constrained age (including reportedly Jurassic intrusive rocks of the Medford dike and Calf Island sill). Twelve of these sites are also targeted for supporting geochronology and ten for geochemistry.