MESOZOIC SYSTEMS ALONG THE PACIFIC 



271 



alaskite 



granite 



quartz monzonite 



granodiorite 



quartz diorite 



5 percent 



4 percent 



47 percent 



33 percent 



11 percent 



This confirms Mayo's observation that quartz monzonite is the most 

 voluminous rock type in the Sierra Nevada where studied petrographi- 

 cally. 



Age of the Batholiths 



The first determination of the age of the Sierra Nevada batholith by 

 isotope methods was made by Larson et al. in 1954. Lead-alpha activity 

 ratios were determined on the accessory minerals zircon, monozite, and 

 xenotime. Seven samples yielded an average age of 100 m.y. Twenty-five 

 samples were run from the batholith of southern California, and these 

 gave an average age of 105 m.y. (Larson et al., 1954). 



A few years later samples were taken by Evernden et al. ( 1957 ) from 

 eight individual intrusions in the Yosemite Canyon area of the Sierra 

 Nevada batholithic complex, plus a pegmatite of one of the plutons and 

 their ages determined by the potassium-argon method. The major plutonic 

 bodies had been mapped by Calkins (1930) and Rose (1957) who had 

 established for the most part the relative ages of the intrusions on con- 

 vincing field evidence. From youngest to oldest the seven plutons are 

 named as follows: Johnson granite porphyry, Cathedral Peak granite, 

 Half Done quartz monzonite, Sentinel granodiorite, El Capitan granite, 

 Gateway granodiorite, and Arch Rock granite. The Hoffman quartz mon- 

 zonite, which is noted to have intrusive relations to the Cathedral Peak 

 granite, was also sampled. The experimental age determinations agreed 

 perfectly with the relative ages determined by geological field relations. 

 The youngest, the Johnson granite porphyry, yielded a date of 82.4 

 ( ± 1-2%) m.y., and the oldest, the Arch Rock granite, 95.3 ( ± 1-2%). The 

 authors from theoretical considerations regard these ages as slightly 

 younger than the true absolute ages of the plutons, but believe any change 

 made ultimately will be in the order of a few percent at most. 



A pegmatite in the Hoffman pluton (83.3^1-2% m.y.) yielded an age 



of 76.9 m.y., and the range from this youngest rock to the oldest is there- 

 fore approximately 18 m.y. This intrusive activity would have occurred 

 according to Curtis et al. (1958) during the Cenomanian, Turonian, and 

 Senonian (see Fig. 17.7) epochs. 



If the series of nine plutons, including a late pegmatite, were intruded 

 during an interval of 18 m.y., a separate intrusion approximately each 

 2 m.y. would have been emplaced. Evernden et al. (1957) review the 

 field evidence to the effect that most of these intrusions were almost com- 

 pletely crystallized at the time die succeeding pluton was emplaced, 

 and thus conclude that crystallization of each would require somewhat 

 less than 2 m.y. 



Fig. 17.11. Plutons and rock types of the Huntington Lake area; Sierra Nevada batholithic com- 

 plex. Direction of pattern lines has no significance. 



