Field mapping, petrography, mineral and whole-rock chemistries have been completed on Precambrian ultramafic and orbicular rocks occurring in the Zuni Mountains, New Mexico. These rock types are surrounded by felsic igneous and metasedimentary rocks, including quartz-feldspar gneiss, quartz monzonite aplite, gneissic quartz monzonite and metarhyolite.
Two small orbicular rock outcrops occur along the contact between quartz-feldspar gneiss and gneissic quartz monzonite. Dark-green, hornblende-actinolite orbicules, having curving crystals of amphibole concentrically arranged about a central point, are set in a pink, quartz monzonite matrix. Many orbicules have been broken or deformed into irregular ellipsoids.
Metamorphosed ultramafic rocks, exhibiting original igneous textures, are present as discrete lenses and pods. Lithologies include: an ultramafic rock consisting of large hornblende crystals (1-3 cm) enclosing round serpentine grains (1-6 mm), interpreted to be poikilitic texture; hornblendite; an ultramafic rock with large actinolite crystals in a matrix of serpentine, amphibole, opaques and chlorite, and chloritite. Original igneous mineralogy consisted of olivine and/or orthopyroxene (now serpentine) and clinopyroxene (now hornblende/actinolite); metamorphic grade reached upper-greenschist to lower-amphibolite conditions.
Ultramafic breccia, found along the margins of some ultramafic lenses, is composed of hornblendite fragments in a pink, granitic matrix. Compositions of amphiboles (Ca29Mg55Fe16) in the breccia and feldspars (albite and orthoclase) in the matrix are identical to those found in orbicules. Amphibolites, bordering many of the lenses and pods, represent hybrid rock between ultramafic and surrounding lithologies, formed by intrusion of felsic igneous bodies.
It is proposed that the orbicular rock resulted from the intrusion of gneissic quartz monzonite into ultramafic bodies. Farthest from the intrusive contact, ultramafic breccias formed; closest to the contact, breccia fragments reacted with the intruding magma to form orbicules. Evidence includes: matching mineral chemistry of amphiboles from ultramafic rocks, breccia and orbicular rock; matching mineral chemistry of feldspars from the gneissic quartz monzonite, breccia matrix and orbicular matrix; and striking textural resemblances between the breccia and orbicular rock.
The ultramafic rocks may represent feeder dikes for komatiitic lava flows, zoned ultramafic plutons, segments of a layered intrusion, basic dikes similar to "cortlandtite", or fragments of an ophiolite. Komatiite feeder dikes can be eliminated as a possible origin for the Zuni Mountain rocks based on texture, mineralogy and bulk chemistry. Further differentiation cannot be made without additional studies involving trace and rare-earth elements and geothermometry.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Albert Masakiyo Kudo
Second Committee Member
Lee A. Woodward
Third Committee Member
Jeffrey A. Grambling
Lambert, Ellen E.. "Geology and Petrochemistry of Ultramafic and Orbicular Rocks, Zuni Mountains, Cibola County, New Mexico." (1983). https://digitalrepository.unm.edu/eps_etds/269