28 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



PINE RIDGE 



CHISTOPHER 

 MOUNTAIN RIDGE 



CHEDISKI RIDOE 



REDWALL LM (M) 



MARTIN FM. (D) 



_^V / J «-7- // ^ MAKTIN I.. 101 ■ <-.■ X • ,>^\V\\V ^-TN. 



■/■ //;wA.:^-KSff!h»J*2*!?ss : wiJ5^. v -\. "MMSgV. TROY >A«I>?Z" OUARTZITE 



70 MILES APP.- 



Fig. 4.4. Restored section across the northern part of Mazatzal Land. After Huddle and Dob- 

 rovolny, 1950. 



project into the Grenville belt or wedge out to the northeast in the Great 

 Lakes region. 



Arizona. The 1350-m.y. orogenic belt is here called the Mazatzal from 

 relations in central Arizona (Fig. 4.4). A correlation of the Precambrian 

 rocks of Arizona by Anderson (1951) is given in Table 4.1, and in it will 

 be seen that the Mazatzal quartzite is regarded as the youngest of a 

 group of old rock units, mostly schists. E. D. Wilson (1939) showed that 



Table 4.1. Correlation of Precambrian Rocks of Arizona (C. A. Anderson, 1951) 



Grand 



Canyon 



(Noble and 



Hunter 



1917; Darton, 



1925) 



Bradshaw Mtns. 



(Lindgren, 



1926) 



Mazatzal Mtns. 



(E. D. Wilson, 



1939) 



Globe 



(Ransome, 



1903) 



Younger 

 Precambrian 



Grand 



Canyon 



Series 



Chuar group 

 Unkar group 



Unconformity 



Apache group 



Apache 

 group 



Orogeny— Intrusion of granitoid magmas 



Older 

 Precambrian 



Vishnu schist 



Yavapai schist 



Mazatzal quartzite 

 Maverick shale 

 Deadman quartzite 



Alder series 



Red Rock rhyolite 



Yaeger greenstone 



Pinal schist 



granite, and thus dated the orogeny and intrusions as post-Mazatzal. 

 the Mazatzal quartzite was folded and faulted prior to the intrusion of 

 He named the orogeny the Mazatzal revolution, and this event now seems j 

 to be dated by the new isotope age determinations, and therefore is ap- 

 plied to the entire belt up through Colorado, Wyoming, and South 

 Dakota. 



It should be noted that the Vishnu schist is 25,000 feet thick where ex- 

 posed in the Grand Canyon of the Colorado, and was originally fine- 

 grained argillaceous sandstones and sandy shales. A sequence of basaltic 

 lavas and tuffs is now represented by amphibolites in which relict pillow 

 and anygdaloidal structures prove the volcanic character. The Vishnu 

 schist is intruded by plutonic rocks that range from quartz diorite to 

 granite. In fact, granite is more widespread in outcrop in Arizona than the 

 host rocks, and therefore the Mazatzal orogeny must be considered, there 

 at least, to be identified with great batholithic intrusions of fairly acidic 

 rock. 



Colorado. The largest exposure of basement crystalline rocks in the 

 Rockies is in the core of the Front Range of Colorado. It consists essen- 

 tially of granite, schist, and gneiss (Lovering and Goddard, 1950). 



The oldest rocks in the Front Range are the schists and gneisses of the 

 Idaho Springs formation, which are highly metamorphosed sedimentary rocks 

 of early pre-Cambrian age. The thickness is approximately 20,000 feet. The 

 hornblende schist and gneiss of the Swandyke hornblende gneiss is overlain 

 by a series of quartzites and quartz pebble conglomerates at least 14,000 feet 

 thick. These formations are all cut by an extensive series of granite intrusives, 

 the oldest of which is a quartz monzonite gneiss. It occurs chiefly in small stocks 

 peripheral to granite batholiths or as a lit-par-lit injection of the older schists 

 and gneisses. Gneissic granite, gneissic aplite, and gneissic diorite are found 

 in abundant but small masses within the metamorphic terrain and are believed 

 to be related to nearby granite batholiths of different ages. 



The earliest of the batholithic granites is the Boulder Creek granite; it is 

 common in stocks and small batholiths in the central part of the Front Range. 

 Its dark-gray color and faindy banded appearance distinguish it from the 

 pink coarse-grained Pikes Peak granite, which is somewhat younger and forms 

 the extensive batholith of the southern part of the Front Range. The appearance 

 and age relations of the Pikes Peak granite are the same as those of the Sherman 

 granite exposed in the large batholith extending from the northern part of 

 the Front Range well into Wyoming. Small batholiths and stocks of the younger 

 fine-grained to medium-grained light pinkish-gray Silver Plume granite are 



