ORIGIN AND EMPLACEMENT OF AN ORBICULAR GRANITE DIKE IN WINSTED, CONNECTICUT

Orbicular granites are unusual granites that host concentrically zoned spherical to ellipsoidal orbicules. A 1.0 to1.5 meter-wide dike of orbicular granite in Winsted, Connecticut is located near the terminus of a Jurassic fault that crosscuts gneissic Precambrian country rock. The dike itself contains no obvious deformational fabric. The dike contains an inner orbicular zone consisting of densely packed feldspar-quartz-biotite orbs ranging from 2 to 3 centimeters wide and 5 to 7 cm long. The orbicules are elongated parallel to the strike of the dike, and in some cases, orbicular boundaries are deformed by contact with neighboring orbicules. Orbicules have medium grey centers (0.5 to 1.5 cm wide) and white rinds (1 to 3 cm wide). Gray orbicule centers consist of large (to 5 mm) alkali feldspar crystals with finer (0.2 to 0.5 mm) quartz and biotite crystals on feldspar grain boundaries. Orbicule rinds consist mostly of coarse radiating elongate feldspar crystals that appear to have nucleated on the surface of the orbicule cores. A finer-grained, mostly orbicule-absent margin (~30 cm wide) occurs on either side of the orbicule-rich dike center. The occurrence of orbicules only in the center of the dike suggests that they were concentrated during flow/emplacement of the dike (the Bagnold effect). The presence of biotite within the core of the orbicules, its relative absence in the orbicule rinds, and the nucleation of quartz and feldspar on the orbicule cores all suggest that cooling was rapid after the temperature of the melt dropped below that of biotite crystallization. The radiating texture of the feldspar crystals in the orbicule rinds is reminiscent of spherulite textures in rhyolitic lava flows. In that setting, the radiating texture is likely to be the result of rapid under cooling. In the orbicular granite studied here, we suggest that spherulite-like orbicular texture is the result of rapid crystallization of the granitic melt as it came in contact with the fracture walls. Rapid cooling, orbicule formation, orbicule migration to the center of the dike, and orbicule-orbicule collision and deformation are likely to have been nearly synchronous.