Large gabbro stock enveloped by a horseshoe-shaped syenite ring dike. Magnetic and radioactivity anomalies and gravity modeling. Sm-Nd and a Rb-Sr isochrons indicate contemporaneous emplacement approximately 405 m.y. ago. Three gabbroic rock types: olivine-plagioclase, olivine-clinopyroxene-plagioclase, and clinopyroxene-plagioclase. Major and trace element variations. The gabbro-syenite trend probably represents differentiation of a tholeiitic basaltic melt near a thermal divide along the critical plane of silica undersaturation. Fractionation took place in a magma reservoir at depth, before intrusion of the syenitic residual liquid into the fractured region between previously intruded gabbro and country rock.--Modified journal abstract.

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