IDAHO BATHOLITH AND THE OSBURN FAULT ZONE 



325 



Setting in Nevadan Tectonic Plan 



It is clear that two great arcuate segments of the Nevadan orogenic 

 belt converge in eastern Idaho (refer again to Fig. 17.13), and just a 

 little south of this junction is the Idaho batholith. The same relation to the 

 Laramide orogenic belt has already been pointed out, although the 

 Nevadan segments are convex westward and the Laramide are convex 

 eastward. The Nevadan segments are also curved more and meet at a 

 more acute angle than those of the Laramide. As previously suggested, the 

 junction area of such arcuate segments of a great orogenic belt may be a 

 favorable place for the rise of great batholiths, but it is difficult even to 

 ; guess why. 



The somewhat similar relation of both Nevadan and Laramide belts to 

 the Idaho batholith does not help in restricting or narrowing down the age 



I of the pluton. 



i 



Relation to Tertiary Sediments 



A fruitful field of research on the age of the Idaho batholith seems to lie 

 in Paleocene and Eocene conglomerates to the east. Certain voluminous 

 I conglomerates in northwestern Wyoming are composed of Beltian quartz- 

 ite boulders and pebbles which are foreign to the formations of the areas 

 : in which they occur. Their only source seems to be the Beltian strata that 

 : crop out along the eastern edge of the batholith in Idaho and western 

 Montana. See the Geologic Map of the United States. Also, Ross ( 1928 ) 

 points out that the Idaho batholith was intruded extensively in the Beltian 

 strata, and that a roof of Beltian rocks, fully a mile thick, has been largely 

 removed. In fact, it was removed before the Oligocene and Miocene 

 lavas and sediments accumulated. The connection between the doming 

 of the quartzites, their erosion, and the formation of extensive con- 

 glomerate deposits nearby seems obvious; the dating of the intrusions by 

 the conglomerates seems a certain procedure. But the extent and age of 

 the various conglomerates east of the batholith are only fragmentarily 

 known, and some of the conglomerates may be made up of boulders that 

 had already composed a former conglomerate. As far as known, the 

 nearest coarse deposit is the Lima conglomerate in southwestern Montana 



which is Paleocene in age (Scholten et al, 1955). The oldest of the 

 extensive conglomerates of the Yellowstone-Gros Ventre-Wind River 

 region is late Paleocene in age, and its boulders have been transported a 

 great distance because of the near-perfect rounding of them. This frag- 

 ment of information suggests very Late Cretaceous or early Paleocene 

 age, again, for the Idaho batholith. 



Isotope Age Determinations 



The absolute age of the Idaho batholith has recently, and with reason- 

 able assurance, been determined by Larson et al. (1954) by lead-alpha 

 activity ratios on the accessory minerals, zircon, monozite, and xenotime. 

 Five analyses yield an average age of 103 m.y. Similar determinations 

 on 7 samples from the Sierra Nevada averaged 100 m.y., and 25 samples 

 from the batholith of southern California gave an age of 105 m.y. Accord- 

 ingly, it may be concluded that the Idaho batholith is very nearly the 

 same age as the Sierra Nevada. Also, a potassium-argon age determination 

 on the Coast Range batholith near Vancouver by Follinsbee et al. ( 1957 ) 

 is reported as 105 m.y., again approximately the same. A few years after 

 Larson et al. samples were taken by Evernden et al. (1957) from 8 individ- 

 ual intrusions in the Sierra Nevada whose age relations had been 

 determined geologically. The samples were run by the potassium-argon 

 method and the ages reported range from 76.9 m.y. for the youngest to 

 95.3 m.y. for the oldest. These ages are a little under the true absolute 

 age, but not more than a few percent, according to the authors. It may 

 follow that when potassium-argon age determinations are made of the 

 Idaho batholith that they will prove appreciably lower than those of 

 the lead-alpha activity ratio method. Since the Idaho batholith is probably 

 composite, the relation of age determinations by different methods is a 

 bit uncertain, especially since the sequence of intrusions in the Sierra 

 Nevada ranges through 18 m.y. 



According to the Holmes B time scale the intrusions dated by the potas- 

 sium-argon method in the Sierra Nevada range through the Albian 

 (uppermost Lower Cretaceous) and the Cenomanian (lowermost Upper 



