Date of Graduation

Fall 2022


Master of Natural and Applied Science in Geography, Geology, and Planning


Geography, Geology, and Planning

Committee Chair

Gary Michelfelder


Assimilation and crystallization are difficult to constrain at magmatic boundaries, including the interactions of magma with the surrounding country rock. The assumption of the relationship between a magma and what it is intruding upon is relegated to homogenous bodies or epizonal plutons. Realistically, wall rock influences chemical heterogeneity and isotopic variance at the outcrop scale and changes depending on distance from the magma-wall rock interface. Here, we present a case study of the 35 Ma Mt. Princeton Batholith and the host Precambrian rocks of the Central Colorado Volcanic Field. We assess chemical heterogeneity by whole rock and mineral trace element and isotope geochemistry, U-Pb zircon geochronology, and mineral assemblages in thin section at varying distances from the interface of the batholith with the wall rock. Two transect locations were selected that provide the best-defined contacts between the wall rock and the Mt. Princeton Batholith containing one sample from the host rock and 7-10 samples from the granite. We present a quantitative model for disequilibrium melting versus crystallization during magma emplacement. We suggest the percentage of zircons with Precambrian age cores included in the granitic body versus magmatic age cores represents the volume of magma affected by the wall rock melting. Granites range in age from 34.1 ± 0.6 Ma to 34.7 ± 0.3 Ma. Precambrian rocks contain two zircon intercept ages, averaging 1600 Ma, and 35 Ma showing evidence of partial resetting, metamorphism, and discordance. Zircon ranges in U-Th ratios from 0.097 - 40.3. Potassium feldspars in the host rock contain Sr contents from 3.9 ppm to 528 ppm and Rb from 4 ppm to 326 ppm. In granites, Sr ranges from 59.4 ppm to 723.9 ppm and Rb from 11.5 ppm to 174.6 ppm. Granite samples nearest to the wall rock contain K-spar inherited from the host where, as distance from the interface increases, these crystals become less prevalent. Modeling trace elements contents and Sr isotopic ratios of potassium feldspar, along with U-Pb zircon geochronology quantifies disequilibrium melting of Precambrian wall rock during emplacement of granitoid composition magmas. These data provide insight into the thermal state of the crust, where a magma partially melted the wall rock, creating a mixing zone at the interface.


Mt. Princeton, geochronology, isotope geochemistry, granite, wall rock

Subject Categories

Geochemistry | Geology | Volcanology


© Loren A. Bohannon

Open Access