IGNEOUS AND TECTONIC PROVINCES IN SOUTH AMERICA 



545 



Bolivia the broad Paleozoic area comprises the eastern and central Cor- 

 dilleras, and the strata are folded but evidently not enough to produce 

 much metamorphism. In Peru the belt is narrower with considerable fold- 

 ing, faulting, and metamorphism, and may include Precambrian rocks. 

 It also includes several large intrusions, one of which is mentioned as a 

 granite (Jenks, 1956). 



In Ecuador the crystalline rocks are highly metamorphosed and form 

 the backbone of the Cordillera Oriental (also called Eastern Andes and 

 Cordillera Real). The types are orthogneiss and paragneiss, mica, garnet 

 schists, also amphibolitic, sericitic, talcose, and graphitic schists, phyllites, 

 and some quartzites and calcareous slates. Also prominent are meta- 

 morphosed granodiorites. Minor amounts of metamorphosed syenite and 

 low-quartz granite are noted. The belt of crystalline rocks is flanked on the 

 east by little or non-metamorphosed Paleozoic and Mesozoic strata, with 

 associated volcanic rocks and Mesozoic (?) granites. The complex is 

 thrust eastward at its eastern margin. ( See section A-A', Fig. 34.5. ) 



Between the Cordillera Occidental, or batholithic belt, and the Cor- 

 dillera Oriental, or older metamorphic belt, is the intercordilleran depres- 

 sion. It may be compared to a huge graben bounded on the east and west 

 by fault zones which dip away from the graben at fairly high angles. 

 Beginning in Miocene time, as far as known, the graben has had large 

 amounts of volcanic materials poured into it. Faulting is believed to 

 have continued intermittently during the accumulation of the volcanic 

 irocks. 



As the two Cordilleras have risen relative to the intercordilleran depression, 

 the volcanoes in and bordering the depression have filled it with vast quantities 

 of predominantly andesitic pyroclastic and flow rocks. At the same time, heavy 

 rainfall and melt water from the snow and ice-clad heights of the Cordilleras 

 have eroded the depression-facing slopes and deposited the resulting clastic 

 sediments in the depression. The huge area flooded by this volcanic and — to a 

 lesser degree — clastic fill makes up by far the greater part of the intercordilleran 

 depression (Lewis et ah, 1956). 



The prominent anticlinorium of Paleozoic and Precambrian rock is 

 for most of its length bounded by reverse faults, and is interpreted to be 

 thrust over the flanking basin sediments on the east and also over the 

 rocks on the west such as the intercordilleran depression in Ecuador and 



the Lake Titicaca trough at the Peru-Bolivia border. These faults delimit 

 the raised zone of older metamorphic rocks, but do not mark the original 

 width of it. 



In Figs. 34.3 and 34.4 highly folded Paleozoic strata are shown to 

 form the core of the postulated Mesozoic volcanic archipelago on the 

 Pacific border of the continent. This picture is built from a few small 

 outcrops, but nonetheless it is as logical a foundation for the Mesozoic 

 volcanic effusives as any. It must be concluded that the width of the belt 

 of Paleozoic folding and metamorphism is much wider than that exposed 

 in the anticlinorium. 



POST-BATHOLITHIC VOLCANIC ROCKS 



Age Relation to Batholithic Belt 



Following the batholithic intrusions and the accompanying folding and 

 faulting a long cycle of erosion removed much of the roof rock and in 

 places cut deeply into the plutons. The surface developed over much of 

 the adjacent Cordillera also. Upon this extensive erosion surface new 

 effusives were spread. The plutonic cycle occurred in most places during 

 Mid- and Late Cretaceous times, and the earliest eruptives are late 

 Eocene, but the main volcanic activity in most places did not start until 

 the Miocene. There was a lapse of time, then, of about 40 m.y. between 

 the plutonic cycle and the beginning of the volcanic cycle. 



Areas of Volcanic Rocks 



The volcanic accumulations may be grouped in three divisions: (1) 

 between Santiago and the Straights of Magellan; (2) southern Peru, 

 Bolivia, northern Chile, and northwestern Argentina; (3) Ecuador and 

 southwestern Colombia. All are confined to the general Cordillera ex- 

 cept in Southern Argentina where flows occur on the foreland. Each 

 of the three areas support a magnificent belt of active or dormant vol- 

 canoes in addition to extensive volcanic fields. (See maps, Figs. 34.1 

 and 34.2.) 



The central division is the largest in areal extent and undoubtedly the 

 largest in volume. It occurs around the bend of the great Cordillera from 



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