Tabulation of radiometric ages--Including unpublished K-Ar and fission-track ages for rocks in southeastern Arizona and southwestern New Mexico
— Richard F. Marvin, C. W. Naeser, and H. H. Mehnert

This tabulation was prepared as an aid to understanding the regional geology of southeastern Arizona and southwestern New Mexico. Figure 1 shows the location of dated samples; Table 1 gives the radiometric age, published reference, latitude, longitude and other information. Tables 2 and 3 contain unpublished analytical data for K-Ar and fission-track ages, respectively. Almost all of the tabulated ages have been calculated or recalculated with decay constants recommended by I UGS Subcommission on Geochronology in 1976; recalculated ages are slightly greater than their published counterparts. A number of new radiometric ages appear in accompanying articles of this guidebook, but they are not included in this tabulation.

 

In recent years, the Denver geochronology labs of the U.S. Geological Survey have determined 36 radiometric ages in con-nection with new geologic investigations and the extension of older studies. These new ages aid in better correlation of vol-canic rocks and some plutonic rocks whose localities are noted on the eastern part of Figure 1. Additional age information is now available to help explain the complex magmatic and tec-tonic history of the gneissic core complex of the Rincon Mountains, shown in the western part of Figure 1.

 

Samples from localities A, 1, 4, 5, 12 and 13 are from igneous units of the Hidalgo Volcanics, a formation that con-tains many extrusive units. Fission-track ages of zircons (other minerals are prophylitized) indicate that andesites and dacites of the Hidalgo Volcanics are of various ages (Late Cretaceous, Paleocene, Eocene or Oligocene). Some of these rocks may be spatially and/or genetically related to intrusive bodies (Thor- man, 1977; Drewes, in press; Thorman and Drewes, this guide-book). Radiometric ages for samples 3, 4 and 2, in the north-ern part of the Pyramid Mountains near Lordsburg, indicate the age of emplacement of the Lordsburg stock and the age of mineralization in the Lordsburg mining district. The stock formed about 59 m.y. ago; the basaltic andesite flow (locality 4) may be a late extrusive phase of the intrusive. Mineraliza-tion in the quartz veins probably occurred about 56.5 m.y. ago as a late phase of the intrusive and extrusive activity (Thorman and Drewes, this guidebook). The felsic dikes are about 52 m.y. old. The dikes and quartz veins are part of the same fracture system but do not intersect.

 

Both intrusive and extrusive igneous activity during the Oligocene is verified by ages for samples at locations, B, C, 7, 9, 13, 21, 22 and 33. These apparent ages are in agreement