The Pecos greenstone belt--A Proterozoic volcano-sedimentary sequence in the southern Sangre de Cristo Mountains, New Mexico
— James M. Robertson and R. H. Moench

Proterozoic igneous and metamorphic rocks are exposed in several approximately north-trending belts in the Sangre de Cristo Mountains of north-central New Mexico and south-central Colorado. In New Mexico, with the exception of the Picuris Range, these rocks received little detailed geologic attention prior to the mid-1970's. Recent mapping has delineated an extensive volcano-sedimentary terrane (hereafter informally called the Pecos greenstone belt) that seems analogous to Archean greenstone belts in the Canadian Shield. The Pecos greenstone belt occupies an area of some 650 km², mainly in the headwaters area of the Pecos River northeast of Santa Fe (figs. 1 and 2). It is defined by a closely interrelated assemblage of metamorphosed subaqueous basalts and locally important felsic metavolcanic rocks, iron-formation, and meta-sedimentary rocks, some of volcanic provenance. The Pecos greenstone terrane is faulted on the west, and is intruded by voluminous plutonic and apparently subvolcanic rock. Although rocks of the greenstone terrane are at least twice folded, variably metamorphosed in the greenschist and amphibolite facies, and intruded by the abundant igneous rocks, fine details of primary sedimentary and volcanic features are preserved locally. These features serve to identify the original nature of the units, but they are not recognizable widely enough to make simple work out of regional mapping. Volcanic and sedimentary rocks both exhibit marked variation in lithology and thickness over short distances. Detailed mapping in areas of continuous exposure allows the construction of local stratigraphic columns (Riesmeyer, 1978), but the delineation of a general stratigraphic succession for the entire greenstone belt is not yet possible.

This paper defines the geologic and geographic boundaries of the Pecos greenstone belt; describes the present state of knowledge of the general geology, petrology, geochronology and economic potential of the terrane; outlines important remaining problems; and speculates on the tectonic setting of the terrane and on a possible faulted extension of the terrane that has a high potential for the occurrence of volcanogenic massive sulfide deposits.