SOUTHERN ARIZONA ROCKIES 



437 



NEVADAN OROGENY (?) 



Post-Permian and pre-Lower Cretaceous folding is recorded in south- 

 eastern Arizona and to the southeast in Coahuila (see Chapter 14). In 

 both places granitic magmas have intruded the folded strata and were 

 exposed by erosion before the Lower Cretaceous beds were deposited. 

 To the west at Ajo, folding ( ? ) and metamorphism preceded the intrusion 

 of the Chico Shunie quartz monzonite. These events seem to correlate 

 with the pre-Lower Cretaceous orogeny in southeastern Arizona. Farther 

 to the northwest in the Artillery Mountains Paleozoic limestone, shale, 

 and sandstone were in part metamorphosed before the Tertiary, at least. 

 No intrusions of possible Mesozoic age are noted there, however. Then 

 in central and western Nevada a long succession of deformational events 

 are documented from late Devonian to the close of Jurassic time. In the 

 Kimmeridgian (latest Jurassic) considerable volumes of granitic magma 

 invaded the folded and thrust-faulted strata. This Late Jurassic orogeny 

 has been classed as early Nevadan in Chapter 17. 



It will be recalled (Chapter 14) that the Coahuila peninsula rose in 

 Kimmeridgian time, and that the Mexican geosyncline took form to the 

 west during the Late Jurassic. It received sediments from a geanticlinal 

 area on the west as well as from the Coahuila peninsula. The rise of the 

 peninsula and geanticline may indicate that both were orogenic belts. 

 This was a time of thrusting and intrusions in central and western 

 Nevada. These coincident relations support the thesis that southern 

 Arizona was a belt of orogeny in the Late Jurassic and that the belt 

 existed as a branch from the main belt in Nevada which continued south- 

 ward into Sonora, Mexico, in the region of the geanticline that lay west 

 of the Mexican geosyncline. See tectonic map of Plate 10. 



IGNEOUS CYCLES AND MINERALIZATION 



Igneous rocks possibly of Palezoic age have been described in two 



places. According to Ettlinger (1928), a diabase in the mountain region 



of central Arizona is intrusive as multiple sills in Cambrian strata but not 



; in any younger Paleozoic strata and, therefore, may be pre-Devonian. 



The diabase extends over 1600 square miles, and the combined thickness 

 of the sills in places approaches a mile. Others have suggested a post- 

 Permian and Cretaceous age. It predates the mineralization of the region. 

 Gilluly ( 1946) regards a hornfels in the Ajo district of the desert region as 

 possibly Paleozoic. It, however, is of andesitic and rhyolitic derivation, 

 unlike the composition of diabase. 



Rutler and Wilson ( 1938 ) list the Juniper Flat stock of the Bisbee dis- 

 trict as post-Paleozoic and pre-Cretaceous, and suggest that the activity 

 may be Nevadan in age. Ransome considered the Sacramento stock at 

 Bisbee and associated metallization also as pre-Cretaceous. As already 

 noted, Gilluly identifies several plutons in the Dragoon Mountains as 

 post-Permian pre-Lower Cretaceous. 



A large number of stocks that range from granite to diorite occur in the 

 mountain and desert regions of Arizona, and all are probably vounger 

 than the Kaibab (Permian) limestone. Of late years they have been con- 

 sidered Laramide in age, mainly upon the argument that they are similar 

 in lithology and their structural setting is similar to other known Lara- 

 mide intrusions of the southwest. Definite proof of Late Cretaceous age 

 is probably not obtainable for most of them. 



A group of the stocks in central Arizona are considered to be cupolas 

 of a major underlying parent pluton called the central Arizona batholith 

 (Ettlinger, 1928). 



In general, the stocks have little plan or pattern in their distribution. 

 In the Superior-Miami-Globe and Morenci-Metcalf districts, however, 

 the intrusive bodies have a general northeastward direction across the 

 mountain region. Likewise, the ore-bearing fissures in these and other 

 districts strike northeastward (Butler, 1938). The mining districts shown 

 on the index map, Fig. 27.2, are a pretty good clue of the distribution 

 of the stocks. Many of the ore deposits of Arizona are due to the 

 mineralizing activity of the magmas of these stocks, especially in the 

 central and southeastern part of the state. 



Lava outpourings are very extensive in Arizona and New Mexico, and 

 some are shown on the index map of Fig. 27.2. Refer also to the Geologic 

 and Tectonic maps of the United States. In part they are younger than the 

 monzonitic intrusions, but in part thev are possibly contemporaneous or 



