Shelburne Falls gneiss dome, Massachusetts

Abstract

The Shelburne Falls gneiss dome, in the eastern Berkshire Hills of northern Massachusetts, consists of amphibolitic metavolcanics, intruded by sills and complex lit‐par‐lit injection‐zones of a quartz‐diorite and its associated volatiles. The dome is surrounded on all sides by the Conway schist, of early Paleozoic age.The dome surface dips outward at gentle angles, and the gneiss‐schist contact is conformable, although an original unconformity is suggested in a few places. Sills or apophyses of quartzdionte in the schist are lacking; neither the distribution of metacrysts nor the mineral composition and texture of the schist show relationships to the gneiss contact. It is concluded that the gneiss complex is older than the Conway schist. On the other hand, both the Conway schist and the gneiss have been silicified near the dome border, and chlorite, muscovite, tourmaline, and a few pegmatites have been introduced near‐this surface. It seems that these changes occurred during, or soon after, the metamorphism of the Conway sediments. At about the same time a few dikes of quartz‐diorite porphyry were intruded into the gneiss and the schist. At least three of them have been tourmalinized, silicified, and micacized.