568 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Kef 



Eogleford 



C Penn Tesnus 



Kbu 



Buda 



Devonian Santiago and Cobollos 



Kdr 



Del Rio 



OP Ordovicion Maravillas and under! 



Krfl 



Georgetown 



Poleoioic undifferentiated 



Ked 

 Kcp 



Edwards 

 Comanche Pk. 



B|B Undifferentiated igneous rock; 



Kgr 



Glen Rose 





Kt 



Tr.n.ty congl. 





K 



Glenrose, Edwo 



ds, and Cretaceous undifferentiated 



Fig. 36.5. The Solitario. Simplified from E. H. Sellards, W. S. Adkins, and M. B. Arick. Un- 

 published map from Bureau of Economic Geology, University of Texas. 



The Terlingua-Solitario region is one of profuse and diversified igneous 

 rocks. According to Lonsdale (1940), there are several hundred masses 

 distinct enough to be mapped in an area of about 400 square miles. They 

 occur as lava flows, plugs or necks, dikes, sills, laccoliths, bysmaliths, and 

 possibly stocks. The largest plutons are laccoliths. Solitario is the largest 

 domed-shaped structure of the group and is strikingly circular. It may be 

 a laccolithic dome (see Fig. 36.5). The igneous rocks of the district in- 

 clude an analcite-bearing series which ranges from melanocratic gabbro 

 to syenite types. Analcite is primary, deuteric, and hydrothermal. Also 



included is an intermediate trachytic and rhyolite group. Most of the 

 varieties are soda-rich. Lonsdale shows the igneous rocks of the Terlingua- 

 Solitario region to be closely related to those of the Spanish Peaks region 

 and also to those of north-central Montana. 



The analcite-bearing rocks obviously are a related series and originated 

 through differentiation which preceded from melanocratic types through 

 labradorite-rich types to syenite (Longsdale, 1940). 



Baker ( 1935 ) has suggested that the uplifted block containing the 

 Solitario dome is underlain by a batholith. In the adjacent sunken block 

 in which nearly all the analcite-bearing rocks occur it is possible that 

 the sinking resulted in rise of magma drawn from the lower and relatively 

 basic part of the batholith. The result would be not a single immediate 

 source of all the analcite-bearing rocks, but a number of differentiating 

 masses in laccoliths and other minor intrusions from which, in the total, 

 a relatively large number of varieties would be produced (Lonsdale, 

 1940). This is much the same arrangement as Larson postulates for the 

 calc-alkalic series of the San Juan volcanic field. 



The Chisos Mountains consists of a number of sharp peaks of intrusive 

 and extrusive rocks. The area is referred to as an uplift, and is com- 

 parable to the Solitario in varieties of igneous rocks and includes alkalic 

 types similar to the Terlingua-Solitario district. 



Alkalic rocks have been penetrated in wells drilled for oil in the 

 adjacent Delaware basin, but a problem exists in determining whether 

 these are Tertiary or Precambrian (Flawn, 1952). 



The west Texas alkalic province extends southeastward well into 

 Mexico, for in the San Carlos Mountains an alkalic suite occurs. Kellum 

 ( 1937 ) describes in the Sierra de San Jose division of the San Carlos 

 Mountains an "alkalic rock complex," a feldspathoid-bearing sill, ijolite 

 plugs, as well as microgranite, quartz diorite, and diorite porphyries. 

 There are also late basalt flows. The porphyries are probably laccoliths. 

 In the Sierra de Cruillas division of the San Carlos Mountains Imlay 

 ( 1937 ) describes microgranite and sills as the most common type of 

 igneous rock. A vogesite sill was noted which is about 90 feet thick and 

 at least 232 miles long. A trachyte sill was also mapped. Basalt of alkalic 

 varieties occurs as a laccolith and as sills and plugs. One plug is an 



