36 



cut the lavas and tuffs of the Excelsior formation consist of much altered 

 basic and also silicic porphyritic types. They are probably contemporane- 

 ous with the extrusions (Muller and Ferguson, 1936). 



Following marine invasions and sharp folding and thrusting more thick 

 volcanic deposits occur, which are of Jurassic age. These rocks were ex- 

 truded during continued crustal unrest, and petrographically cannot be 

 distinguished from the Triassic Excelsior volcanics. For further details 

 refer to Chapter 6 and 17. Also examine map, Fig. 36.1 (symbol, pre- 

 batholithic volcanic rocks) for distribution. 



IGNEOUS PROVINCES IN 

 WESTERN UNITED STATES 



UGEOSYNCLINAL PROVINCE 



The region west of the Antler orogenic belt in Nevada and California 

 jvas one of considerable volcanic activity in middle and late Paleozoic 

 ime, especially in the Permian, and a thick assemblage of strata accumu- 

 ited typical of the eugeosyncline. Volcanism persisted into the Mesozoic, 

 nd in the mid-Triassic 12,000 feet of strata, chiefly pyroclastics and lavas, 

 ccumulated to form the Excelsior formation. The rocks range in com- 

 'osition from andesite through quartz latite to rhyolite with andesite 

 robably predominating. Keratophyres with secondary albite have been 

 lentified but probably have limited distribution. Certain intrusions which 



BATHOLITHIC PROVINCE 



Repeated Paleozoic, Triassic, and Jurassic orogeny occurred in the 

 eugeosynclinal province before the deformed complex was invaded by 

 the great batholiths. See Figs. 17.2 and 17.7. It has been pointed out that 

 the Calaveras formation ( Mississippian ) is more metamoqmosed than the 

 Mariposa (Jurassic) in places, but it is clear that the Mariposa was 

 sharply folded and thrust-faulted before the granodiorite intrusions. This 

 has been regarded as a climactic orogeny immediately preceding the 

 intrusions, but in the Sierra Madre del Sur of Mexico and in the South 

 American Andes such an orogeny is either not evident or was of milder 

 intensity, and the rocks into which the batholiths were emplaced are 

 believed to be Paleozoic strata deformed and metamorphosed in late 

 Paleozoic time. 



The Sierra Nevada plutonic mass is a composite of many separate 

 intrusions each of batholithic size. In the area of Yosemite National Turk 

 the individual batholiths made their ascent at about 2-million-year inter- 

 vals over a period of 18 million years (Evernden et a!., 1957). The 

 process of intrusion took place during Albian and Cenomanian time of 

 the middle Cretaceous. Much of the rock is of forceful intrusive nature 

 but considerable stoping, migmatization, and contamination of the 

 primary magma occurred in places. 



As shown in the cross section E-E' of Fig. 34.1 the batholithic belt in 

 central Baja California is about 175 miles (260 kilometers) wide, and in 

 the California-Nevada region it has about the same width, if the satellite 



553 



