GNEISS DOME AT SHELBURNE FALLS, MASSACHUSETTS

Abstract

The Shelburne Falls gneiss dome, in the eastern Berkshire Hills of western 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 by the Conway schist of early Paleozoic age. The principal rock types are described. The dome surface dips outward at gentle angles, and the gneiss-schist contact is conformable, though an original unconformity is suggested in a few places. Sills or apophyses of quartz diorite in the schist are lacking. Neither the distribution of metacrysts nor the mineral composition and texture of the schist shows zoning with regard to the gneiss dome. It is concluded that the gneiss complex is older than the Conway schist, but that both the Conway schist and the gneiss have been silicified near the dome border, and chlorite, muscovite, tourmaline, and a few thin 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 3 of them have been silicified, tourmalinized, and micacized. Tightly compressed, isoclinally folded amphibolite tracts in the gneiss complex, with axes of folds and lineation in various directions, contrast with gently flexed or mildly folded Conway schist in the vicinity of the dome. Axes of folds and lineation pitch off the dome for the most part, though not everywhere in the steepest directions. Inward-dipping funnel joints near the dome border, and steep fractures radiating outward from it, attest to the mechanical expansion of the crust during the doming. The association of amphibolitic metavolcanics with soda-rich igneous rocks, in the gneiss complex, resembles that of certain gneissic terranes in southeastern Vermont and southwestern New Hampshire more closely than the microcline-rich gneisses of the crest of the Berkshire Hills.