U-PB ZIRCON DATE OF THE CA. 1.44 GA VIRGINIA DALE ANORTHOSITE, NORTHERN COLORADO

Two large anorthosite complexes are exposed north of the Colorado-Wyoming line in the Laramie Mountains: the ca. 1433 Ma Laramie Anorthosite Complex and the geographically associated but significantly older Horse Creek Anorthosite Complex (1761.5 ± 2 Ma). The 1439 to 1430 Ma A-type Sherman batholith intrudes these anorthosite complexes. The undated Virginia Dale ring complex and comingled intrusion straddles the state line and has lithological and Nd isotopic similarities to the Sherman batholith (Vastek and Kolker 1999). Southeast of the ring complex in Colorado, a small, little-known anorthosite body was mapped and published in 1989 by W.A. Braddock, D.H. Eggler, and T.R. Courtright. The Virginia Dale Geologic Quadrangle Map GQ-1616 shows the small anorthosite body to the southeast of the ring dike and to the west of U.S. 287.

We have informally named the body the Virginia Dale anorthosite. Braddock et al. described it as a quartz-bearing anorthosite consisting of andesine plus minor hornblende, biotite, and quartz. Most of its perimeter is in contact with Early Proterozoic biotite granite gneiss, but on the southern side the anorthosite is cut by the Middle Proterozoic Silver Plume Granite.

We collected several geochronology samples to assess the relationship between the Virginia Dale anorthosite and the various well-dated anorthosites of the Laramie and Horse Creek Anorthosite Complexes. A large sample (60 pounds) yielded numerous anhedral, magmatic zircons for dating by the CA-ID-TIMS high-precision method in the U-Pb geochronology lab at the University of Wyoming. A robust, 4-point weighted mean, ²⁰⁷Pb/²⁰⁶Pb date of 1440 ± 1.6 Ma was determined from concordant analyses, which is a bit older than the Laramie Anorthosite Complex. The date is at the older limit of the Sherman batholith dates, 1439 to 1430 Ma but it is well within the range of ages of the 1.4 Ga granites throughout Wyoming and Colorado. The new date is interpreted as not just the age of the Virginia Dale anorthosite but potentially also the ages of the Virginia Dale ring complex and all the associated coarse-grained, A-type granites exposed nearby.