Approximately 7 to 8 square miles of Precambrian rocks are exposed in the vicinity of the Monte Largo Hills in north-central New Mexico. The Precambrian rocks are bounded mainly by Pennsylvanian sedimentary rocks in both fault and noncomformable contact.
The area is made up largely of quartz-feldspar gneiss, hornblende gneiss, and quartzite. Quartz-feldspar gneiss is the most abundant rock type. Mica schist and sillimanite hornfels occur in minor amounts. Other Precambrian rocks include medium-grained granite, splite and pegmatite dikes, and quartz veins. A minor amount of migmatite is present as a result of the intimate association of quartz-feldspar gneiss and granite.
The metamorphic rocks are believed to have been derived largely from sedimentary rocks. The quartz-feldspar gneiss probably represents and feldspathic sandstone or subgraywacke; the hornblende gneiss, a graywacke; the quartzite, a rather pure quartz sandstone.
The Precambrian rocks have been subjected to three types of metamorphism: regional, thermal, and retrograde, in that order. The mineral assemblage hornblende-andesine-quartz in the hornblende gneiss indicates that regional metamorphism is of the almandite-amphibolite facies. Randomly oriented sillimanite in pelitic hornfels is indicative of either the hornblende-hornfels facies or the pyroxene-hornfels facies of thermal metamorphism. Retrograde alteration of biotite to muscovite, mangetite, and chlorite is indicative of the greenschist facies of metamorphism. Sericitization of plagioclase and sillimanite is widespread. There is some evidence that retrograde metamorphism was accompanied by introduction of potassium resulting in the formation of muscovite. The three types of metamorphism are believed to represent one period of metamorphism.
It is possible that granitization by the introduction of potassium to form microline has contributed to the formation of the quartz-feldspar gneiss.
Alkalic and related rocks younger than the plutonic rocks include a melteigite sill, carbonatite, and a breccia of explosive origin. The melteigite and carbonatite are similar to rocks associated with the alkalic stock at Iron Hill, Colorado.
The average strike of the foliation of the metamorphic rocks is northeast and the average dip is about 700 to the southwest.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
J. Paul Fitzsimmons
Second Committee Member
Wolfgang E. Elston
Third Committee Member
New Mexico Geological Society
Monte Largo, New Mexico, Precambrian Rocks, Quartz-feldspar, Hornblende, Quartzite
Lambert, Paul Wayne. "Petrology of the Precambrian Rocks of Part of the Monte Largo Area, NM." (1961). http://digitalrepository.unm.edu/eps_etds/187