- UW Department of Geology and Geophysics
The Virginia Dale intrusion is a granitic and monzogranitic composite intrusive body at the southern extent of the Sherman batholith in southeastern Wyoming and adjacent areas of Colorado. Within the study area, at the margin of the intrusion, mafic rock (diorite) is commingled with felsic rock (monzogranite) and intermediate rocks. Resulting features include pillows (some with zones of felsic or mafic enrichment at their margins), enclaves, mantled quartz grains, and alkali feldspar megacrysts. Intermediate rocks interpreted as hybrids include: (1) granodioritic hybrid; (2) main-phase hybrid; (3) granitoid hybrid; and (4) mafic hybrid. Each of the hybrids shows distinct textural and mixing relations.
Mixing and hybridization models were evaluated using major-element, trace-element, and isotopic data. Major-element compositions of the hybrid rocks generally plot along simple mixing lines between mafic and felsic rocks. Incompatible element concentrations of the intermediate rocks are similar to, or greater than, concentrations in the felsic end-member. Trace-element data suggest that the variety of intermediate rocks resulted from chemical diffusion in addition to bulk mixing of heterogeneous end-members. Mafic rocks show a possible differentiation trend, in which the most primitive members are cumulates. The more evolved members show progressive LREE and incompatible element enrichment, growth of a negative europium anomaly (to Eu/Eu* = 0.61), and a correlated decrease in Mg-number (to 0.35). Virginia Dale isotopic data (ϵNd = 0± 1) are similar to values for the adjacent Sherman batholith, suggesting that both of these bodies formed by partial melting (at 1.43 Ga) of ca. 1.8-Ga crust in the Colorado province. Initial 87Sr/86Sr values are more heterogeneous, perhaps due to chemical diffusion.