Orbicular rock located on the west side of the Sandia Mountains, New Mexico, at W. long. 106° 29' 30", N. lat. 35° 13' 04", was first reported by Fitzsimmons (1966). Recently Thompson and Giles (1974) published a paper describing.three types of orbicular rock in the locality which they believed to have been formed by a metasomatic process. Without knowledge of the work of Thompson and Giles, this thesis was initiated as a detailed field, petrographic, geochemical and geochronologic study of these orbicular rocks.
Four types of orbicular rock, including another previously unreported occurrence of orbicular granite about 2 km north of the locality cited above, have been analyzed by wet chemical methods. Two K-Ar dates on biotites from one orbicule (1333.6 +/- 27.9 m.y.) and from the biotite monzonite host rock (1312.6 +/- 27.6 m.y.) in close association with the orbicular rocks have been obtained in addition to a 1550 + SO m.y. Rb-Sr whole rock date on a large sample of one type of orbicular rock.
The metasomatic or metamorphic origin as proposed by Thompson and Giles (1974) is not supported in this study for several reasons. 1. Thompson and Giles (1974) believe that the biotite monzonite is an inclusion with which the granite reacted to metasomatically produce the orbicular rocks. The field and geochronologic data indicate that the biotite monzonite is not a xenolith, but rather a biotite-rich magmatic segregation which coexisted with the Sandia granite melt. 2. The shell geometry of orbicules (Leveson, 1963) which is used by Thompson and Giles (1974) to support a metasomatic origin is not a definitive criterion for origin. Indeed, single orbicules in the Sandias have shell spacings indicative of an "igneous" origin for the inner shells and a "metamorphic" origin for the outer shells. 3. A careful examination of the chemistry o-2 the shells indicates that a metasomatic diffusion process for the formation of orbicular rocks, a s suggested by Leveson (1963), is highly improbable. The data show that such a diffusion process would require repeated reversals in ion concentration gradients during shell development. Ions which diffuse inward at one stage must diffuse outward during the next stage and so forth.
For the above reasons an igneous origin is proposed. This theory of origin is supported by the following criteria: (1) nonnal compositional zoning of the plagioclase shells, (2) identical shell layering and shell mineralogy within each orbicule type without regard to core composition, (3) consistent plagioclase composition (i.e. An31) of the outermost shell regardless of orbicule type or core material, (4) age data, and (S) liquid-like, defonnational features found in one type of the orbicular rocks.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Douglas Gridley Brookins
Second Committee Member
Albert Masakiyo Kudo
Third Committee Member
J. Paul Fitzsimmons
Enz, Robert David. "Geochemistry and Petrology of the Orbicular Rocks, Sandia Mountains, New Mexico." (1974). https://digitalrepository.unm.edu/eps_etds/267