The Sandia Mountains, which are immediately northeast of Albuquerque, New Mexico, consist mostly of Precambrian "granite" [technically speaking it is a granodiorite/quartz monzonite], as well as some adjacent metamorphic rocks. Overyling the granite is the Madera Formation, a fossiliferous limestone of Pennsylvanian age. There is a profound erosion surface (unconformity) between the granite and the basal part of this limestone.
A very unusual and interesting rock, called a orbicular granite, is locally well known from about mid-way up the west face of the Sandia Mountains. It occurs in a somewhat dike-like (parallel sides) exposure, about 30 feet wide and perhaps several hundred feet long. The orbicular granite has very distinctive black-and-white orbicules. Most of them are about 2 inches in length and 1 inch wide. The longest ones are up to 5.5 inches, and the widest ones are 2.75 inches (note: not the same orbicule). The typical orbicule consists of a core rock of granite or feldspar, surrounded by a shell of black biotite, which is, in turn, surrounded by a shell of white oligoclase. The latter consists of radiating crystals. Some of the core rock appears to be biotite, but this is most likely just a function of the angle of the exposed surface.
|A large piece of orbicular granite from the Sandia Mountains. Some of the centers|
of the orbicules are granite, others are feldspar, and many others appear to be biotite.
The scale (in centimeters) is 15 cm (6 inches) in length.
|A hand specimen of the orbicular granite. The oligoclase crystals in the white shell are|
arranged perpendicular to the crystals in the underlying biotite shell.
The Sandia granite is one of the most age-dated granites in North America. Based on K/Ar and Rb/Sr age-dating, the orbicules and surrounding granite are approximately 1,300 to 1,350 million years old. Many researchers have done work on the orbicular rock, and the overwhelming majority of them suggested an igneous (magma) origin. Only a very few researchers have suggested a metamorphic (metasomatic origin) that involved reactions between xenoliths (hand-sized inclusions) and water-rich or vapor-rich magmatic fluids. The conditions of the orbicule formation have not been determined completely.