Volcanic rocks of the Cienega area, Santa Fe County, New Mexico
Authors: Sun Ming-Shan , Baldwin Brewster 
Published: March 2023

Detailed mapping of the Cienega area, which is 5 sq mi and is situated 14 mi southwest of Santa Fe, NM, indicates the following sequence of volcanic and sedimentary units of Cenozoic age: the Eocene and Oligocene Galisteo Formation, of sandstone and red mudstone, is overlain conformably by andesite breccia. Those units were domed by intrusion of augite monzonite. The andesite breccia is correlative with the second igneous period of the Cerillos area, 5 mi to the southwest; the monzonite is correlative with the fourth igneous period.

Erosion exposed the monzonite. Extrusion of calcic latite flows and breccia, glassy latite breccia, and the Cieneguilla limburgite followed in sequence. The Cieneguilla limburgite consists of flows and tuff-breccia of limburgite and olivine basalt, possibly of Miocene age. Augite basalt, a conglomerate of volcanic pebbles, a hornblende andesite dike, a vent breccia, and a felsite dike are probably older than the limburgite, but their position in the sequence is uncertain. During the making of the Rio Grande trough, rocks of the Cienega area were faulted and tilted 10 degrees northeastward. The rocks were then eroded.

In late Pliocene or Pleistocene time, the area was covered by at least 100 ft of sand and gravel of the Ancha Formation. Basalt flowed over the western part of the area, diverting the ancestral Santa Fe River. Hills of monzonite and of limburgite stood above the gravel- and basalt-covered plain. In Quaternary time, the Santa Fe River cut as much as 400 ft below this plain, revealing the Tertiary sedimentary and volcanic units. The geologic map of the area, scale 1:15,840, indicates the places where samples were collected for laboratory study.

Petrographic descriptions are given for each of the volcanic rock types noted above. The definition of monzonite is discussed on the basis of the molecular norm of various rocks. The approximately equal amounts of orthoclase and plagioclase in the definition of monzonite and latite are here considered to be the equivalent of a normative orthoclase-plagioclase ratio of 30:70. Six spectrographic analyses show that the minor constituents-vanadium, chromium, cobalt, and nickel-are inversely proportional to the silica content of the rocks. Vanadium is stored in pyroxene, instead of in olivine, which is rich in basic rocks. In basic rocks, the amount of nickel exceeds that of cobalt. The cobalt-nickel ratio of calcic latite is 2.67; the cobalt-nickel ratio of glassy latite and monzonite is 1.

About 80 rock samples were powdered and fused to form artificial glasses, and the index of refraction was determined for each glass. Complete chemical analyses were made for seven of these samples, and the silica content only was determined for 14 additional samples. Of the 21 points on the curve of silica content versus index of refraction, 19 are within 2.5% silica, or 0.010 in index, and 2 points are within 5%, or 0.020 in index. The index of refraction aided in assigning some samples to their proper geologic units and in approximating the silica content of the samples. The index depends on silica content and on other constituents and factors.