34. 



IGNEOUS AND 

 TECTONIC PROVINCES 

 IN SOUTH AMERICA 



CHILE AND ARGENTINA 



Geosyncline 



Two references are most significant for a general understanding of the 

 Andean geology of South America, viz., Handbook of South American 

 Geology, Geol. Soc. Am. Memoir 65, 1956, edited by W. F. Jenks, and 

 Ban der Sudamerikanischen Kordillera (Gebriiden Borntraeger, Berlin) 

 by Heinrich Gerth, 1955. The Cordillera of Chile and western Argentina 

 marks essentially the site of a previous geosyncline, particularly the eugeo- 

 synclinal division. (See Fig. 34.1.) Its Paleozoic history is not well known, 

 but as far as a great igneous and geosynclinal cycle is concerned we may 



start with the late Permian and early Triassic, when continental condi- 

 tions probably prevailed. During this time voluminous extrusions of 

 keratophyre and quartz porphyry occurred. 



. . . These extrusions are pierced by granites which are the intrusive phases 

 of the lavas. Later, the sea advanced from the west, eroding the volcanic rocks 

 and depositing a transgressive series which has at its base the products of the 

 destruction of the volcanics which in turn pass upward into shale with a 

 marine fauna. This transgression marks the beginning of the Andean geo- 

 syncline. Later the keratophyre extrusions were renewed, with a more basic 

 composition than previously, and flows partially filled the marine basin. Plant- 

 bearing shales were deposited. But all these episodes were transitory because 

 the ocean transgressed again during the Norian (late Triassic), with the deposi- 

 tion of thick layers of shale. Later, continental conditions returned, perhaps be- 

 cause of tectonic movements whose nature is unknown. The topograph v 

 formed was then destroyed during the Rhaetian (latest Triassic) when a surface 

 was prepared for the Liassic (early Jurassic) transgression (Cristi. 1956. 

 p. 197). 



The Triassic volcanic rocks are over 12,000 feet thick in the Frontal 

 Cordillera of Mendoza but thin toward the east. 



Late Triassic volcanism continued into early Jurassic time but the 

 distribution of the eruptives is possibly limited to southern Atacama 

 and northern Coquimbo. 



... In the rest of Chile andesitic volcanics seem to be lacking in the Lias. 

 However, in the Coast Range of Aconcagua and Valparaiso, the Upper Lias 

 sediments contain thick keratophyre flows and tuffs. Apparently similar condi- 

 tions are found in the Argentinean Cordillera. It is interesting to note that this 

 type of extrusion is not known in the region north of Atacama; this proves that 

 the keratophyre extrusions, which probably began during the Lower Triassic in 

 an area of enormous size, become more and more restricted. At the same time 

 acidity of the flows diminished. This phase of volcanism ended in the late 

 Liassic. 



During the Upper Dogger (mid-Jurassic), andesitic extrusions covered almost 

 all the area occupied by the western part of the geosvncline: but. at least in 

 the Coast Range and the central zone, it seems that before these lavas were 

 deposited many important tectonic movements occurred, possibly in the form 

 of block faulting, since in some places the deposits lie on Triassic and in others 

 on Liassic formations. 



We know little about the mechanism of these extrusions, but judging by 

 some masses of andesite which pierce the Triassic or Liassic, and by the 

 abundance of pyroclastic materials in the series, probably they were produced 

 by volcanoes of the central type, which must have been elevated above the sea 



537 



