SILURIAN TECTONISM IN THE WESTERN NEW ENGLAND APPALACHIANS

Discontinuous sills of granite along the eastern margin of the Berkshire massif were interpreted by Ratcliffe and Hatch (1979) as syntectonic anatectic melts that intruded a Taconic thrust zone separating Middle Proterozoic basement from the structurally overlying Late Proterozoic Hoosac Formation. The granite sills vary in thickness from 1 to 100 m and are found in both the basement rocks and the Hoosac Formation. Exposed sill contacts are sharp and the granite is weakly to moderately foliated. Five samples from the Becket and Otis 7.5” quadrangles are geochemically homogeneous. They plot in the volcanic arc granite field on commonly used tectonic discrimination diagrams. The concordia plot of SHRIMP ages for one sample shows a strong cluster of young rims that give a weighted average 206Pb/238U age of 432 +/- 3 Ma (n=11). The older core 207Pb/206Pb ages from this sample range from ca. 960 to 1250 Ma. The concordia plot for another sample also has a strong cluster of young ages that give a weighted average 206Pb/238U age of 434 +/- 5 Ma (n=8). The older core 207Pb/206Pb ages for this sample range widely from ca. 790 to 1170 Ma. We interpret the 432 +/- 3 Ma and 434 +/- 5 Ma ages as the time of crystallization of the granite of Becket Quarry. The older cores are xenocrystic and their ages indicate that the granite sills were produced by, or at least contaminated by, partial melting of Middle Proterozoic basement rocks. The arc geochemical affinity of the sills may be inherited from partial melting of Middle Proterozoic arc-related rocks. Alternatively, the sills may record a limited magmatic pulse above a west-dipping Silurian subduction zone under the Laurentian margin during an interval dominated by back-arc basin extension. The age of the sills, together with their proximity to a fault zone that juxtaposes younger on top of older rocks, suggests that the eastern margin of the Berkshire massif is a Silurian normal fault, possibly related to extension and the opening of the Connecticut Valley trough.