New K-Ar dates for the Precambrian of Pinal, Gila, Yavapai and Coconino counties, Arizona

Abstract

In a recent paper Damon and Giletti (1961) reviewed the available information concerning the Precambrian basement rocks of the Colorado Plateau and adjacent areas. The reader may refer to this paper for a more detailed discussion and for additional references. In the following discussion, we will merely make new data available to the Arizona-New Mexico geologic fraternity. It is our intention to discuss in detail these and other results at a later date. In a short note in the Bulletin of the Geological Society of America, Giletti and Damon (1961) stated that, through the year 1960, "no published evidence indicates an age greater than 1550 m.y. for any Arizona rocks." Since that time, Silver and Deutsch have reported an age of 1660 m.y. for the Johnny Lyon granodiorite (1961), and more recently Silver (1961) has presented evidence for the occurrence of the major metamorphism of the Pinal schist in the Dragoon quadrangle (Cochise County) at very near 1700 m.y. Lanphere and Wasserburg (1962) report Rb/Sr ages of about 1800 m.y. on muscovites and potash feldspars from pegmatites in the Precambrian rocks of the Grand Wash cliffs south of Lake Mead, Arizona. Furthermore, Damon, et al. (1962) have obtained K-Ar ages of 1680 m.y. on Precambrian metamorphics in the CaborcaAltar area of Sonora, which is about 150 km. southwest of the border town of Nogales. In Table I of this paper, a K-Ar date of 1660 m.y. is reported for the Madera granite in the Pinal Mountains of Gila County. Thus, there is