Summary: The Lower Cambrian Weymouth Formation at Nahant, Massachusetts, consisting of interbedded pelitic and calcareous rocks, was intruded by the Nahant Gabbro. The predominant metapelitic mineral assemblage of the contact aureole is quartz-muscovite-chlorite-magnetite-ilmenite. The calcareous hornfelses exhibit a varied mineral assemblage, and in some cases the variation can be spatially related to the intrusive. A number of cross-cutting thin mineral veins, many containing prehnite, are characteristically associated with these calcsilicate rocks.

The minerals have been analysed by electron microprobe and this work indicates the presence of a possible solvus in the Fe³⁺-Al epidote solid solution series. At the physicochemical conditions of the formation of the Nahant hornfelses, the ferric mole fractions of coexisting epidotes are 0.49 and 0.98.

Comparison with experimental work shows that the conditions of the contact metamorphism were T ≃ 500°C, P_total ≃ 2 kb, and X_CO{in2 ≃ 0.2. However, the present assemblages are the result of a later low-grade regional metamorphism, the ultimate product of which was prehnite.