Precambrian formations and Precambrian history in Cochise County, southeastern Arizona
— Leon T. Silver

Precambrian rocks of southeastern Arizona typically are exposed in isolated masses in a number of the tectonically complex ranges of the southern Basin and Range province. Preservation of original sedimentary and igneous textures and structures is commonly excellent, but the combination of extended Precambrian histories, discontinuous exposure and variable overlay of Mesozoic and Cenozoic deformation, magmatism and metamorphism provides major challenges for regional Precambrian correlations and historical interpretations. For all of the problems, however, the early work of F. L. Ransome (1903, 1904, 1915, 1919, 1923, and elsewhere) and N. L. Darton (1924, 1925) successfully established a basic Precambrian stratigraphic framework for this region which subsequent studies have refined but not replaced.

In this brief article primary attention is given to the problems of the Precambrian record in Cochise County, in the southeast corner of Arizona, with some references to the larger regional setting involving adjacent areas of Arizona, New Mexico and Sonora, Mexico. In addition to available literature sources, this report draws on unpublished, currently active investigations of geochronology, petrology and geochemistry in Precambrian terranes of southwestern North America by the author, colleagues and his students.

The leading modern field investigator of the Precambrian formations of Cochise County has been J. R. Cooper, U.S. Geological Survey. He mapped the Precambrian rocks of the Dos Cabezas, Chiricahua and Swisshelm mountains and the Circle J and Fisher hills (Cooper, 1959, 1960). Together with colleagues, he published a detailed study of Precambrian stratigraphy and structure in the Dragoon quadrangle (Cooper and Silver, 1964) which is the current reference study for the Precambrian stratigraphy and structure of southeastern Ari-zona (fig. 1). Other important field studies include those by Gilluly (1956) in the Dragoon Mountains; Sabins (1957) in parts of the Chiricahua and Dos Cabezas mountains; Erickson (1968) in the Dos Cabezas Mountains; Hayes and Landis (1964) in the southern Mule Mountains; Hayes and Raup (1968) in the Huachuca Mountains; Creasey (1967) in the Whetstone Mountains; and Drewes (1975) in the Happy Valley quadrangle, on the east side of the Rincon Mountains.

The summary geochronological information offered here is based largely on extensive studies of U-Pb isotope systematics in cogenetic zircon populations from igneous and meta-morphic components, utilizing techniques and interpretations developed originally for, and applied to, rocks of this region (Silver, 1963, 1964; Silver and Deutsch, 1963). These methods have shown remarkable internal consistency and compatability with observed field relations in this region, whereas other geo-chronological techniques (whole rock and mineral 87 Rb-87Sr and mineral 40 K-4° Ar) commonly have reflected the wide-spread isotopic disturbances induced upon Precambrian materials during the Phanerozoic. The decay constants on which the U-Pb apparent ages are based are the values recently adopted by international convention [A238 U=0.155125 x 109; X235 U=0.98485 x 10-9; 238-2 fur 25 U=137.88] from the work of Jaffe and others (1971).