Ash-flow stratigraphy in the Mattapan Volcanic Complex, greater Boston, Massachusetts

The late Proterozoic Mattapan Volcanic Complex south and west of Boston consists primarily of ash-flow tuff. Nine analyzed samples are all high-silica rhyolites with trace-element patterns typical of calc-alkaline suites. Based on petrographic characteristics and trace-element concentrations these are subdivided into the older Twin Pine Tuff and the younger High Rock Tuff. The Twin Pine Tuffranges from vitric tuff containing only 5 percent crystals on the east side of the Stony Brook Reservation to crystal tuff averaging 20 percent crystals (plagioclase > quartz > perthite) in Westwood and Sherborn. The crystal tuffs become more crystal-rich (from 15 to 30 percent) and more plagioclase-rich upward. Flattened shards and pumice lapilli give rise to foliation in most outcrops. Twin Pine tuffs cluster at higher values of Nb/Zr, Y/Zr, and Ce/La than the High Rock Tuffin Needham, Newton, and the west side of the Stony Brook Reservation. This member is a massive or columnar-jointed crystal tuff containing 25 to 35 percent crystals (plagioclase > quartz > perthite) and rare relic pumice. Plagioclase increases upward. Both Twin Pine and High Rock Tuffs show major-element variations consistent with observed petrographic trends. The high-silica and pyroclastic character of these rhyolites, their compositional zonation, and their total thickness (measured in kilometers) allies them with Tertiary caldera complexes throughout the American West. Younger intrusives that regionally crosscut and engulf the Mattapan ash-flow tuffs may represent magmatic resurgence after caldera collapse. These are tentatively assigned to the Westwood Granite.