Metamorphosed ultramafic rocks in the Bronson Hill anticlinorium, central Massachusetts

Ultramafic lenses occurring in the Middle Ordovician Partridge formation represent one of a wide variety of late Precambrian to Devonian metamorphosed ultramafic rocks in the Bronson Hill anticlinorium. Most of the Partridge formation lenses are olivine-orthopyroxene hornblendites. Their mode of occurrence, mineralogy, bulk chemistry and metamorphism are discussed in detail. Analysis of three models to determine their origin and emplacement favours an origin as Middle Ordovician picritic lava flows rather than tectonically emplaced tectonized ophiolite fragments or products of hydrothermal sea-water alteration of tholeiitic volcanic rocks.-M.S.