36 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



apparently unconformably by metasedimentary and volcanic rocks, and in 

 one place by a gabbro sheet (?). The granitic intrusion therefore, cor- 

 related in age with the Grenville and Piedmont orogenies, and the meta- 

 sediments and volcanics presumably with the Keweenawan series of the 

 Lake Superior region. The Mazatzal orogenic belt appears to separate the 

 Texas Precambrian assemblage from the Grenville, and hence, the most 

 natural tectonic tie of the Texas assemblage appears to be with the Pied- 

 mont (Fig. 4.3). 



Crystalline Piedmont 



A broad belt of crystalline rocks extends from Alabama and Georgia 

 northeastward along the Atlantic margin of the continent to New Jersey, 

 and its relation to the Appalachian Mountains will be explained in some 

 detail in Chapters 8 and 9. In summary, its rocks are now believed to be 



Precambrian and early Paleozoic in age, and to have been metamorphosed 

 and intruded particularly during the Taconic and Acadian orogenies of 

 Late Ordovician and Late Devonian ages, respectively. Age determina- 

 tions on the rocks of the Piedmont indicate two ages, namely, an older 

 one of Grenville age and a younger one of Paleozoic age. In fact, in one 

 sample the zircon grains yielded an age of 1050 m.y., and the feldspars 

 an age of 300 =*= m.y. (Wetherill et al., 1959). It is reasoned that this 

 means an early orogeny in which the zircons were created, and a late 

 orogeny in which the feldspars were formed but the zircons of the 

 early orogeny left unaltered. 



The distribution of dates so far published is shown on Fig. 4.3 and a 

 comprehensive compilation and interpretation of Precambrian trends and 

 orogenic belts of North America by Gastil (1960) is reproduced in Fig. 

 4.9. 



