Nahant Gabbro and gabbro at Salem Neck

Labradorite-pyroxene gabbro, hornblende gabbro and hornblende diorite. Intrudes CAbw. Nahant Gabbro intrudes Early Cambrian Weymouth Formation at Nahant and thus is younger than gabbroic or dioritic rocks intruded by Late Proterozoic Dedham Granite or Topsfield Granite. Unrecognized equivalents to Nahant Gabbro at Nahant and at Salem Neck may be present within unnamed gabbro and diorite units shown on MA State geologic map (Zen and others, 1983). Mapped as three facies at Nahant, following usage of Bell (1977): pyroxene gabbro, olivine gabbro, and quartz diorite. Biotite from olivine gabbro was analyzed by Rb-Sr and K-Ar methods and yielded an Ordovician age (450 +/-13 Ma by Rb-Sr methods, 483 +/-21 Ma by K-Ar methods). Quartz diorite phase may be a more mafic phase of Dedham Granite (Wones and Goldsmith, 1991).
State Massachusetts
Name Nahant Gabbro and gabbro at Salem Neck
Geologic age Ordovician
Lithologic constituents
Major
Igneous > Plutonic > Dioritic > Dioritehornblende diorite
Igneous > Plutonic > Gabbroic > Gabbrolabradorite-pyroxene gabbro, hornblende gabbro
Comments Part of Milford-Dedham Zone (Tertiary and Older Rocks). Secondary unit description per MA007.
References
NGMDB product
Counties Essex