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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.