DISEQUILIBRIUM MELTING OF THE CONTINENTAL CRUST DURING EMPLACEMENT OF THE MT. PRINCETON BATHOLITH, CENTRAL COLORADO VOLCANIC FIELD

Disequilibrium melting of the crust during intrusion changes magma chemistry and, in turn, creates epizonal plutons. The growth and emplacement of magmatic plutons are complex, and many petrologic models consider plutons as zoned or homogenous bodies separate from the intruded host rock. However, magmatic bodies vary in mineral assemblage and texture during emplacement, storage, and crystallization. This project uses U-Pb zircon geochronology to quantify disequilibrium melting of Precambrian wall rock during emplacement of granitoid composition magmas. We present a focused study of the 35 Ma Mt. Princeton batholith and the host Precambrian rocks of the central Colorado volcanic field to examine small-scale compositional change during emplacement of the magma. A primary goal of this study is to provide a greater understanding of the thermodynamic relationship between magma and crust during intrusion and storage leading to better models of magma storage. We test the hypothesis that the thermal state of the crust and melt evolution of mineral phases can be determined during magma intrusion by quantitatively modeling changes in radiogenic isotopic ratios with distance away from the magma-wall rock interface. Two transect were selected that provide the best-defined contacts between the wall rock and the Mt. Princeton batholith for U-Pb zircon geochronology. Twelve samples were selected for U-Pb zircon age by laser ablation ICP-MS. Core and rim analysis was used to analyze 75-100 zircons per sample from the granite and 25-30 zircons from the wall rock. An initial age of zircon rims averaged ~35-36 Ma with a wide range of Precambrian ages within the cores. We suggest feldspar trace element geochemistry and Precambrian U-Pb zircon ages reflect compositional and isotopic heterogeneity in granitic magmatic systems is directly inherited by wall rock melting during magma emplacement. The statistical percentage of Precambrian cored zircons versus magmatic age cores defines the volume of magma influenced by wall rock melting. Therefore, changes in isotopic ratio with reflect consumption of mineral phases in the melt reaction. Preliminary conclusions suggest a significant volume of wall rock melting and recrystallizing in the intruding pluton over time indicated by the range in Precambrian ages in inherited cores.