IGNEOUS AND TECTONIC PROVINCES IN SOUTH AMERICA 



5-39 



bottom, spreading their lavas partly as submarine and partly as terrestrial 

 flows. These lavas and pyroclastics repeatedly filled large areas of the basin. 

 They were subjected to marine erosion, and conglomerate and limestone were 

 deposited on the marine terraces. Finally the filling became so thick that much 

 of the basin acquired continental characteristics, except in the eastern zone, 

 where sediments continued to be deposited until early Malm (Upper Jurassic) 

 time (Cristi, 1956). 



The earlier keratophyres gave way to Jurassic andesites which occurred 

 as flows, "porphyritic" tuffs and welded tuffs, and andesite breccias and 

 conglomerates (boulders are generally andesite porphyry). According to 

 C. Lomnitz of the University of Chile ( personal communication ) some of 

 the keratophyres mentioned in the literature are probably spilites, and 

 pyroclastics predominate over lavas. 



Volcanism continued into earliest Cretaceous time with the accumula- 

 tion of andesite breccia conglomerates and red sandstones derived from 

 the volcanic rocks. Marine sediments are extensive along the Chilean- 

 Argentinian border, and it is thought that the intertonguing volcanic rocks 

 graded into a volcanic chain along the western coast. 



The Santa Cruz basin (also called Magellan geosyncline) on the south 

 lacks Late Triassic volcanic rocks, but in the Jurassic intense volcanism 

 broke out there, and a thick series of keratophyre and andesite flows 

 accumulated. These are called the "Serie Porfirica." (See Fig. 34.1 and 

 section D-D', Fig. 34.5. ) 



. . . The Serie Profirica of the Magellan region has frequently been likened 

 to the Triassic keratophyres of Central Chile and Argentina. Although the two 

 series show great petrographic similarity, they are not synchronous since accord- 

 ing to modern studies by Argentinian geologists, the Serie Porfirica starts during 

 the Jurassic and ends during the Lower Cretaceous. Another important differ- 

 ence between the two extrusive aggregates is that the Mesozoic extrusions of 

 the Andean geosyncline changed from quartz-keratophyres to andesites, whereas 

 in the northern part of the Magellan geosyncline the acidic character was 

 maintained during the whole interval, and only in one place do a few un- 

 important andesites appear. 



On the other hand, formation of both basins was preceded by extensive 

 keratophyre eruptives. 



The rocks above the Serie Porfirica in the Magellan region consist of dark 

 fine-grained sediments with a phyllitic aspect and include marly clay shales and 

 graywackes. Radiolaria are abundant in the lower beds. A few basic dikes cut 



the lower strata. In the Cordillera of the Brunswick Peninsula and Tic i ia del 

 Fuego this series must be several thousand meters thick (Cristi, 1956). 



These seems to be little development of a miogeosynclinal division of 

 the geosyncline; the volcanic sequences and interbedded sediments thin 

 to the shelf area of the foreland with volcanic rocks making up part of 

 the much thinner sequences. 



Batholithic and Metamorphic Belt 



As seen on the map of Fig. 34.1 the entire coastal zone of Chile is made 

 up of a belt of metamorphic and batholithic rock. From Valparaiso south- 

 ward a metamorphic rock zone stretches along the shore almost to the 

 Straits of Magellan. The batholithic zone in this segment lies inland or 

 east of the metamorphic belt but gradually transgresses the metamorphic 

 zone and comes to the coast just north of the Straits of Magellan. As the 

 combined belt veers eastward through Tierr-a del Fuego, the metamorphic 

 belt appears on the inside or to the northeast. The reality of a great 

 batholith is not questioned. In some cross sections it is shown underlying 

 the whole Cordillera, from the Coast Range through the high Andean sys- 

 tem. On the Geologic Map of South America (1950) it is 75 miles wide 

 just east of the Gulf of Corcovado and is almost continuous from Val- 

 paraiso southward through Tierra del Fuego. It has thinned to about 25 

 miles at Valparaiso and from there extends as a narrow belt another 400 

 miles northward. It then becomes discontinuous and is represented by 

 scattered intrusions into southern Peru. From Valparaiso northward it is 

 intrusive into the eugeosynclinal sediments, mostly Late Triassic and 

 Jurassic volcanics. 



The relation of the batholiths to the metamorphic rocks has not been 

 clearly established. In places the metamorphic rocks are intensely folded 

 Paleozoic strata, but the orogeny may be Permian or earlier, at least in 

 places, and not an immediate prelude to the later batholithic intrusions. 

 Extensive gneissic zones in the Coast Range near Valparaiso are probably 

 migmatites of the batholith. 



Briiggen ( 1934 ) has very carefully analyzed the phenomena at the con- 

 tact between the batholith and the country rock, which is generally the 

 Serie Porfirica, and he has concluded that the gneissic appearance of the 



