540 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



batholith in places near the contact is due to migmatization. No doubt, in 

 numerous places a certain amount of migmatization has taken place, but 

 generally the gneissic aspect consists of a folded primary structure, in- 

 jected by veins during the latest stages of magmatic consolidation. 



Both the gneissic appearance and the migmatization are much stronger in 

 the Coast Range, probably because in this range the lower levels, where 

 magmatic phenomena could develop with greater efficiency, are more accessible 

 to observation. For this reason the batholith has until recendy been considered 

 very old. But Briiggen demonstrated through his analysis of numerous outcrops 

 that the intrusion cannot be older than Early Cretaceous, because the strata 

 of this age which are near the batholith are always affected by thermal meta- 

 morphism. Besides, the Paleozoic and Triassic conglomerates, which are rela- 

 tively abundant in Coquimbo, contain no pebbles that could have been derived 

 from the Andean batholith; the pebbles are aplitic granite containing albite or 

 microperthite. Such rocks are subordinate in the batholith, which is mostly 

 tonalite and granodiorite. However, in the Coast Range the first phases of the 

 intrusion might well have been tied to the orogenic movements which occurred 

 in the Jurassic (Cristi, 1956). 



Although the great piles of volcanic rock were intruded, a zone of the 

 eugeosyncline along the east side of the batholithic zone was left free of 

 intrusions. The layers of volcanic rock, here many thousands of feet thick, 

 were tilted so as to dip eastward and from this great monocline in the 

 region east of Santiago the high peaks of the main Andean Cordillera 

 were carved. 



Petrographically the main batholith ranges from granodiorite to tonalite. 

 Fairly extensive bodies of granite are known, especially aplitic granite. 

 Gabbro and hornblendite are listed as present and are said to be basic 

 derivatives, presumably of the parental dioritic magma. Diorite porphyry 

 and dikes of lamprophyre, aplite, and especially kersantite and spesartite 

 are mentioned. True pegmatites are uncommon (Cristi, 1956). 



PERU, BOLIVIA, ECUADOR, AND COLUMBIA 



Geosyncline 



Geosynclinal sedimentation is known to have become established in 

 Middle Ordovician time in Peru approximately in a north-south basin in 



the site of the present Andes. The basin extended, at least, as far west as 

 the present shoreline. Silurian strata have not been observed but Middle 

 Devonian strata are known in places. (See map, Fig. 34.5 section B-B'.) 

 In central Peru, Middle Pennsylvanian beds rest disconformably on the 

 Middle Devonian. In southern Peru, Permian beds rest disconformably on 

 older Paleozoic hard, micaceous shale. Again in central Peru, Permian 

 conglomerate and red beds rest in strong angular unconformity on older 

 Paleozoic rocks in various conditions of metamoq^hism. In southern 

 Peru, marine Permian ( and possibly Carboniferous ) beds cover contorted 

 and metamorphosed older formations. Mississippian of Pennsylvanian 

 continental deposits rest unconformably on older rocks in northwestern 

 and central Peru. Absence of known Upper Devonian and marine Missis- 

 sipian strata in Peru is a further suggestion of orogeny and uplift begin- 

 ning at the end of Middle Devonian time. 



In central Peru late Paleozoic orogeny must have begun at the close of the 

 Middle Pennsylvanian. Continental elastics and volcanics of Permian age here 

 rest disconformably on the Pennsylvanian sequence (Jenks, 1956). 



Permian time was marked in places by marine transgressions and by 

 the deposition of 2300 feet of "red beds and conglomerates" apparently 

 of continental origin. In central and southern Peru the Permian volcanic 

 rocks attain great thicknesses. 



The close of the Paleozoic in Peru was marked by strong orogeny. Permian 

 granites and associated quartz veins cut thick folded and faulted Paleozoic 

 metamorphics in northwestern and probably in southern coastal Peru. There 

 was intense volcanic activity and general emergence of the Andean zone in 

 upper Permian time. Evidently the whole of Andean Peru was land from some 

 time in the Upper Permian until at least the beginning of the Upper Triassic 

 (Jenks, 1956). 



The history thus related is responsible for the belt of metamorphic rocks 

 shown on the map, Fig. 34.2, which stretches from northernmost Chile to 

 northwestern Peru. Its present borders are probably due to later orogeny. 

 A similar belt of metamorphic rocks forms the Cordillera Oriental of 

 Ecuador. Its paragneisses and paraschists probably represent Paleozoic 

 and perhaps some Precambrian sediments deposited in an Andean geo- 

 syncline. 



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