IGNEOUS AND TECTONIC PROVINCES IN SOUTH AMERICA 



543 



the mainland side is portrayed. The volcanic arc and eugeosyncline are 

 about 200 miles wide. In California and western Nevada the same has a 

 width of about 400 miles. Permian volcanic rocks are abundant in both 

 places. 



Batholithic Belt 



The batholithic belt extends up the coast from Chile to northwest Peru, 

 and the great plutons form much of the western slopes and foothills of 

 the Cordillera Occidental. In northern Peru the batholithic belt swings 

 northeast across Peru, following the Mesozoic geosyncline. According to 

 the Geologic Map of South America (1950) the plutons are separate and 

 small in Ecuador, and in Colombia the belt becomes so inconspicuous 

 1 that its identity, at least on the map, is problematical. 



The rocks of the great coastal batholith of Peru are, in order of abun- 

 dance, granodiorite, tonalite, granite, and diorite. Others in small volume 

 , are quartz monzonite, monzonite, syenite, and gabbro. Wide, deeply 

 eroded parts of the batholith appear to be fairly homogeneous in compo- 

 sition, but apically truncated parts show a wide range of petrologic types 

 (Jenks, 1956). 



The main part of the batholith was intruded, apparently, in early Upper 

 Cretaceous time. Lower Senonian and even Turonian strata have been 

 intruded and metamorphosed, but younger ones have not been affected. 

 Lower and Middle Cretaceous rocks in Peru contain abundant volcanics, 

 jbut when the batholithic intrusions ocurred, no further volcanism is 

 recognized through the Maestrichtian, Danian, and Paleocene to the late 

 Eocene. Abundant volcanic rocks appear to be largely Miocene, Pliocene, 

 and Quaternary in age. 



Anticlinorium of Pre-Mesozoic Rocks 



A belt of Paleozoic and Precambrian rocks extends from the Argentine 

 'border northwestward through Bolivia and lengthwise through Peru 

 almost to Ecuador. It is lacking for about 100 miles and then at the 

 southern Ecuadorian border commences again and extends through Ecua- 

 dor and nearly through Colombia. In disconnected areas it is present in 

 western Venezuela. ( See map, Fig. 34.2, and cross section, Fig. 34.5. ) In 



CRETACICO: FACIES VOLCANICA 



JURASICO-TRIASICO: FACIES VOLCANICA 



Fig. 34.4. Idealized restoration of Mesozoic sedimentary and tectonic divisions in northwestern 

 Peru. Reproduced from Fischer, 1956. 



