PRECAMBRIAN TECTONIC PROVINCES 



35 



PRECAMBRIAN 

 STRUCTURAL 

 TRENDS 



MINERAL 

 PROVIN 



DATE 

 CES 



Fig. 4.9. Precambrian structural trends (left map) and mineral date provinces (right map) of North 

 America. Reproduced from Gastil, 1960. 



ments and basic flows, sills, and dikes. The basalts have been described 

 as tholeiitic in both belts (Turner and Verhoogen, 1951). The signifi- 

 cance of tholeiitic basalt is discussed in Chapter 33, and the occurrence is 

 believed to be evidence that the belts formed under similar tectonic 

 settings. Both are on the inside (toward the continent) of master orogenic 

 belts involving extensive metamorphism and great batholithic intrusions. 

 According to this interpretation the Keweenawan belt should mark ap- 

 proximately the inner front of the Grenville orogenic belt or province. 

 Regarding the succession in Texas, it is possible that the Swisher 



gabbroic terrane and parts of the Wichita igneous terrane arc Keweena- 

 wan equivalents, and that the metasedimentary and volcanic (rhyolite) 

 terranes are Huronian or somewhat older than the Keweenawan. 



Texas Precambrian Rocks 



In Texas and southeastern New Mexico a subsurface study of well 

 samples penetrating the Precambrian has enabled Flawn (1956"> to 

 delimit several rock assemblages, which he calls terranes I Fig. I s . The 

 basement rock is a granite dated about 100 m.y. old, and this is overlain 



