Westwood Granite

Westwood Granite - Light-gray to pinkish-gray, fine- to medium-grained granite. Intrudes Zdgr. Proterozoic Z Westwood Granite forms small lenses of light-colored granite intruding Proterozoic Z Dedham Granite and older rocks. Occurs within Dedham batholith. Some rocks resembling Westwood, such as those exposed in Plymouth quarries near Weymouth, have been mapped as part of Dedham Granite on MA State Geologic Map (Zen and others, 1983). Extensive intrusion breccias occur at contacts of Westwood with older mafic rocks. Contacts with Dedham are commonly abrupt; dikes of Westwood cut Dedham and rare inclusions of Dedham are found within Westwood (Chute, 1966). No reports of cobbles of Westwood within Roxbury Conglomerate; therefore, it is possible that Westwood is intrusive equivalent of Mattapan Complex that underlies Roxbury and that Westwood was not exposed to erosion at time of deposition of Roxbury. Also may be an intrusive equivalent of Lynn Volcanic Complex. A somewhat questionable Rb/Sr whole-rock age of 579 +/-28 Ma (Fairbairn and others, 1967) obtained from Westwood indicates it may be younger than Dedham and may be about same age as Mattapan (Wones and Goldsmith, 1991).