A new occurrence of orbicular rock is located in the Sandia Granite at W. long. 106° 25' 30", N. lat. 35° 04' 55" close to the granite-gneiss contact near Tijeras Canyon, Sandia Mountains, New Mexico. The orbicules, set in an aplite-pegmatite granitic matrix, have magmatic and metamorphic cores surrounded by a white plagioclase shell and one or two salmon-colored potassium feldspar shells. The outcrop occurs in two dike-like masses with thicknesses of 1 to 4 m trending N. 5° W. and N. 35° E. for exposed lengths of 16m and 46 m, and is surrounded by biotite syenodiorite and biotite monzonite.
Theories for the formation of previously discovered orbicular rock in the Sandia Mountains are in conflict. Enz (1974) and Enz, et al. (1979) suggest formation in an aqueous fluid where nucleation occurs on available seed material (e.g., xenoliths present in the Sandia Granite) . Thompson and Giles (1974) suggest that the biotite host rock surrounding the orbicules is a xenolith that reacted with the granitic magma creating conditions for orbicule formation.
In the Tijeras Canyon area, the formation of orbicules can best be explained by an igneous process. The model proposed in this paper for the formation of the orbicules involves a water- or vapor-rich fractionation of Sandia Granite melt and subsequent crystallization on available seed material (xenoliths and biotite host rock). Data supporting this model are: (1) all orbicules are surrounded by plagioclase of similar composition (An28-29) regardless of core type, (2) microcline perthite in orbicule shells suggests crystallization above the solidus with unmixing on cooling, (3) microcline perthite and microcline in the matrix suggest the matrix started to form above the solidus and continued to crystallize below the solidus after the shells had formed, (4) complex microcline perthite grain boundaries in the shells occupied by quartz suggest an increase in water concentration as crystallization proceeded, (5) chemical trends suggest the orbicular rock and Sandia Granite are related, (6) stable oxygen isotope δ180 values for minerals from the orbicules are consistent with an igneous origin , (7) the coarse grain-size of the microcline perthite in the shells and the aplite-pegmatite texture of the matrix suggest water- or vapor build-up in the melt, (8) the presence of radiating "cracks" in the potassium feldspar shells filled with matrix suggests the shells were hard before the matrix was completely solidified, and (9) the presence of large amounts of biotite suggests a hydrous environment.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Rodney C. Ewing
Second Committee Member
Douglas G. Brookins
Third Committee Member
Fourth Committee Member
J. Paul Fitzsimmons
Albuquerque Gem and Mineral Club, New Mexico Geological Society, Sun Oil Company
Affholter, Kathleen. "Petrogenesis of Orbicular Rock, Tijeras Canyon, Sandia Mountains, New Mexico." (1979). https://digitalrepository.unm.edu/eps_etds/251