New Mexico Geological Society Annual Spring Meeting — Abstracts

Salinas Peak is the highest (2,733 m) and most prominent (1,105 m relief) peak in the San Andres Mountains. Bachman and Harbour (1970) show that the peak is the thickest part of a sill (490 m thick) made up of microgranite or alaskite. The sill extends at least 7.4 km to the north and 10.8 km to the southwest of the peak. The underlying Paleozoic rocks and contact with the Proterozoic rocks are dipping to the west-northwest about 12 degrees whereas the overlying Paleozoic rocks above the sill dip to the west as much as 24 degrees.

Two specimens of the sill were taken from exposures along the road to the top of Salinas Peak—one from a road cut excavated in jointed rocks about 5 m below the weathered ground surface and a second exposure at the top of the peak. Despite sampling what appeared to be the least weathered rocks in the road cut, the microgranite looked somewhat altered and many of the borders of the joints exhibited liesegang oxidized iron staining. The specimen from the top of the peak had megascopic (2-3 mm in diameter) muscovite that provided a good candidate for ⁴⁰Ar/³⁹Ar age determination.

Backscattered electron (BSE) images acquired during electron microprobe analysis of the road-cut sample show a typical igneous texture of uniformly fine-grained size of 100 microns or less. The sample is composed of an equigranular mix of quartz, feldspar and mica. The feldspar is dominantly potassic feldspar (orthoclase) with minor Na-rich plagioclase (albite). A fine-grained micaceous phase is ubiquitous throughout the sample. Much of this phase appears altered, although some fresh-appearing mica is also present. That the sample has undergone some alteration is reinforced by the presence of calcite, which appears to be replacing orthoclase in some areas of the sample. Quantitative chemical analyses performed on both fresh-appearing and altered areas of the micaceous phase yield a muscovite composition that is chemically essentially identical. The difference appears to be that the altered mica is slightly more hydrous, suggesting that although alteration has added some water to the mica structure, the major cation chemistry of the mica is essentially unchanged.

Large, fresh muscovite yielded a well defined ⁴⁰Ar/³⁹Ar plateau age of 34.45 ± 0.04 Ma that is interpreted to record the age of intrusion. Whole-rock ⁴⁰Ar/³⁹Ar analysis yields a disturbed age spectrum with a younger bulk age of 31.49 ± 0.12 Ma that likely records argon loss related to deuteric alteration within the cooling sill or later subsurface weathering adjacent to joints from infiltrating meteoric water.

References:

1. Bachman, G.O., and Harbour, R.L., 1970, Geologic map of the northern part of the San Andres Mountains, central New Mexico: U.S. Geological Survey, Miscellaneous Investigations Map I-600.