Bulletin-30—Precambrian Geology of the Picuris Range, North-Central New Mexico

By A. Montgomery, 1953, 89 pp., 4 tables, 2 figs., 9 plates, 1 index.

In the course of his operation of the Harding pegmatite mine, a wartime source of tantalum, lithium, and beryllium minerals, the writer recognized the need for detailed field and laboratory study of the enclosing metamorphic and igneous rocks of the Picuris range southwest of Taos, NM. Most of the metamorphic rocks plainly were of sedimentary origin, but metamorphosed igneous rocks also were recognized. Later igneous rocks, only slightly metamorphosed, were intruded into the metamorphic terrane; the notable pegmatites of the area are associated with these rocks. All of these crystalline rocks are older than overlying Pennsylvanian strata, and are interpreted as being of pre-Cambrian age.

A wide variety of quartzites, phyllites, schists, and gneisses is exposed and it was recognized that the continuity of outcrop permits a reasonable interpretation of stratigraphic succession that might provide geologic column of pre-Cambrian rocks with which similar rocks in other parts of northern NM might be compared. Of particular interest to students of metamorphic rocks and to crystal collectors as well, is the widespread occurrence of the aluminum silicates, andalusite, sillimanite, and kyanite, together with the hydrous iron-aluminum silicate, staurolite. The distribution of these and other minerals has permitted the designation of metamorphic zones in the Picuris Range. Overlying and enclosing formations of Paleozoic and Tertiary age were studied only insofar as was necessary to interpret the structure and more recent history of the range.

All important rock outcrops were located with Brunton compass. Wherever helpful, altitude readings were taken with aneroid barometer and recorded. More than 2,300 outcrops were located and studied. Numerous rock specimens were collected, and 300 thin sections from these were studied.

This report embodies the results of field mapping done during the period 1947–1949, and of laboratory studies made in the Department of Geology at Harvard University. The report was submitted during November 1950, in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Harvard University. The field work occupied 2.5 summer seasons from 1947–1949. Areal geology was plotted in the field on aerial photographs purchased from the Soil Conservation Service and having an approximate scale of 2 inches to the mile, then transferred to the base map which is a photostatic compilation of four Soil-Conservation planimetric sheets derived from aerial photographs and having a similar scale. Without topographic control it was impossible to map rock contacts with precision. The USGS topographic sheet, Taos and vicinity, on a scale of 1 inch to 2 mi, covers only the northeast corner of the range.