Abstract
Orbicular rocks that occur within biotite-rich Precambrian granite of the Sandia Mountains in central New Mexico are of three types: (1) multishelled orbicules with alternating biotite- and plagioclase-rich shells, (2) plagioclase orbicules with or without a discontinuous biotite shell near the orbicule margin, and (3) orbicules with plagioclase cores surrounded by thin concentric bands of finely crystalline biotite alternating with plagioclase. Cores of the orbicules consist of fragments of biotite monzonite, plagioclase, or hornfels. Petrographic data on fragment reactions during orbicule formation, an aplite dikelet that cuts the orbicule zone, spacing of orbicule shells, and chemical analyses suggest that these orbicular rocks formed by reactions between xenoliths and magmatic fluids during crystallization of the granite.
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