Geology and U-Pb geochronology of Proterozoic rocks in the vicinity of Socorro, New Mexico
— Samuel A. Bowring, Kent C. Condie, and Ward Sumner

The purpose of this report is to summarize the results of reconnais- sance mapping and U-Pb geochronology for the major exposures of Proterozoic rocks in the vicinity of Socorro, New Mexico. The areas of Proterozoic outcrop discussed here are shown in Figure 1 and include the Lemitar, Magdalena, and Chupadera Mountains. U-Pb zircon ages are reported for three granites and two felsic metavolcanic units in these areas.

Published geologic reports concerning Proterozoic rocks of the Mag- dalena (Condie and Budding, 1979; Sumner, 1980; Loughlin and Koschmann, 1942) and Lemitar (Condie and Budding, 1979; Mc- Lemore, 1980) Mountains have implied simple stratigraphic successions upon which tectonic models have been based. Recent work on Proterozoic rocks of the Pedernal Hills (Armstrong and Holcombe, 1982) and the Manzano (Grambling, 1982; Bauer, 1982, 1983) and southern San gre de Cristo (Grambling and Codding, 1982) Mountains have documented complex structural histories involving multiple episodes of deformation. In light of these studies, we have reexamined the outcrops of Proterozoic rocks in the Socorro area to evaluate this apparent dif- ference in structural style and to establish a chronologic framework of magmatism to compare with other areas of central New Mexico.

In the three major outcrop areas discussed here, geologic mapping has been primarily lithologic; detailed structural analysis has not yet been attempted. However, it is important that in all areas of Proterozoic rocks discussed in this report, structural complexity precludes any discussion of stratigraphy or apparent thicknesses of units at this time and we limit ourselves to discussing general lithologies and preliminary structural observations.

U-Pb analyses were done at the University of Kansas using an au- tomated 22.5 cm radius, single-filament mass spectrometer. Zircon analyses (Table 1) followed the general method of Krogh and Davis (1973). Analytical blanks range from 0.6 to 1.2 nanograms total Pb. Corrections for non-radiogenic Pb were made using model Pb compositions of Stacey and Kramers (1975). All data reduction and age calculations were done using the constants recommended by Steiger and Jager (1977). Concordia intercept ages were derived using a York (1969) least-squares fit for the discordia lines and calculating the in- tercept ages from the mean slope and the -± 1 sigma slope. Uncertainties are reported at the one sigma uncertainty level.