Beverly Syenite

The Beverly syenite is exposed on Salem Neck, south of the Cape Ann granite (No. 133), as a discontinuous strip along the coast from Beverly to Manchester, on the northern part of Great Misery Island, and around the coast north of Marblehead. A number of small islands between Marblehead and Manchester may also be of the same syenite (Clapp, 1921, Plate 2). The principal rock type is a syenite of variable grain size and composition, with nepheline syenite confined to dykes. The syenites are divided by Clapp (1921) and earlier workers into pulaskites, nordmarkites, umptekites etc. The typical syenite contains perthite, arfvedsonitic amphibole, commonly forming poikilitic crystals, brown biotite, aegirine-augite, often zoned, and accessories. Trachytic dykes are abundant and dykes of nepheline syenite, originally divided into diorites and foyaites, are up to 5 m thick and occur mainly at Salem Neck but are also represented along the northerly shore sections and the islands, including Great Misery Island. The nepheline syenites are trachytic rocks of phenocrystal and groundmass perthite, albite, prismatic nepheline, interstitial sodalite, amphibole, a little biotite and accessories. In the vicinity of Salem Neck gabbros and diorites have been invaded by syenite and nepheline syenite with the production of hybrid rocks including 'essexites' with vermiform nepheline-feldspar intergrowths (Clapp, 1921, p. 124). Modal and mineralogical data are available in Toulmin (1964) and rock analyses and detailed petrography in Clapp (1921).