NEW GEOCHRONOLOGICAL AND GEOCHEMICAL CONSTRAINTS ON MAGMATISM IN MIDDLESEX FELLS AREA, SE NEW ENGLAND

In southeastern New England, a series of Neoproterozoic plutonic, subvolcanic, and volcanic units occur in the Avalon Terrane. While earlier researchers grouped these igneous bodies based on regional correlations, recent studies, particularly those employing high-precision dating methods, have identified significant petrological, compositional and temporal variations within these units. We present new field mapping (Ridge, 2024) along with CA-ID-TIMS U-Pb zircon ages and whole-rock geochemical data from the Middlesex Fells area, offering a more detailed characterization and distinction of rocks that were previously mapped as part of the Dedham Granodiorite and Lynn Volcanics. The Spot Pond Granodiorite yields zircon ages of 609.08±0.24 and 609.11+0.22 Ma, linking it to the Dedham Granodiorite. New mapping documents that the Lynn Volcanic Complex in this region consists of a range of volcanic and subvolcanic units. The Boojum Rock Tuff Member, a dacitic tuff, has an age of 596.35±0.21 Ma, while a rhyolitic flow from the Straw Point area in Stoneham yields an age of 595.27±0.34 Ma. Subvolcanic plutonic units previously lumped with the Dedham now appear to be related to the Lynn. The granitic Lawrence Woods Granophyre displays two age populations of 598.13±0.27 Ma and 596.77±0.54 to 596.50+0.53 Ma while the tonalitic Rams Head Porphyry yields an age of 596.24±0.21 Ma. In the northern Fells and areas further north and northeast the Stoneham Granodiorite has an age of 595.14+0.17. Major element geochemistry displays a calc-alkaline trend, while trace element patterns reveal enrichment of Large Ion-Lithophile Elements (such as K and Rb) and depletion of High Field Strength Elements (such as Ti, P, and Ta), collectively suggesting a subduction zone environment.

Ridge, J.C., 2024, Bedrock Geology of the Middlesex Fells Reservation and Adjoining Parks and Preserves in Malden, Medford, Melrose, Stoneham, and Winchester in Middlesex County, Massachusetts, USA: Map and Explanation, 245 p., 1:4600 scale map (last update February 21, 2024). http://sites.tufts.edu/fellsgeology/