The geochronology of some stratified metamorphic rocks in northeastern Massachusetts

Abstract

Analyses of U and Pb in zircons and Rb and Sr in whole rocks have been made on stratified metamorphic rocks in northeastern Massachusetts. The Rb–Sr results show an age of about 450 Ma, which is interpreted as a resetting age. This would indicate the major period of metamorphism in the area. Zircons from the Fishbrook Gneiss give an upper intercept age of 730 ± 26 Ma, which is interpreted as the age of volcanism that produced the Fishbrook Gneiss sequence. The upper intercept of the Westboro Formation detrital zircons is 1511 ± 22 Ma and is interpreted as the age of the source for the Westboro sediments. The lower intercepts of the Fishbrook (289 Ma) and the Westboro (278 Ma) zircons suggest an episodic Pb loss correlated with the thermal event in southeastern New England recognized by other methods. The intercept ages of the Shawsheen Gneiss (2042 and 517 Ma) are more difficult to interpret, but also suggest an old source area. This source area is tentatively suggested to be northwest Africa.