21. 



of the batholith are blanketed by Tertiary strata, mainly Miocene vol- 

 canic rocks, and it is generally recognized that the batholith is much 

 larger than that exposed and shown on maps. 



IDAHO BATHOLITH AND 

 THE OSBURN FAULT ZONE 



EXTENT 



The Idaho batholith extends from the vicinity of Boise northward 

 through the center of Idaho into Montana, and has an area of over 16,000 

 square miles. Plutons of batholithic dimensions occur in the narrow north- 

 ern end of Idaho, as if to link the main Idaho batholith to the Loon Lake, 

 Colville, and Nelson batholiths. See Fig. 17.13. Smaller batholiths and 

 stocks also occur nearby in western Montana, namely, the Boulder bath- 

 olith near Butte, the Philipsburg stocks near Philipsburg, and other un- 

 studied and unnamed batholiths to the south and southeast. 



Most of the western, all of the southern, and part of the eastern borders 



COMPOSITION 



Composition of Main Mass 



According to Boss (1928), the Idaho batholith is composed mainly of 

 quartz monzonite, although marginal fades are commonly granodiorite. 

 In northern Idaho north from Pend Oreille Lake large plutons are com- 

 posed of granodiorite, quartz-monzonite, and granite, and are regarded 

 sufficiently similar to the Idaho batholith to be connected with it geneti- 

 cally and to bridge it to the Nelson batholith in British Columbia ( Boss, 

 1928). 



Later Anderson (1942) found the marginal faeies to have been diorite 

 originally, with only minor amounts of quartz, and that it was subse- 

 quently altered by widespread rising solutions rich in silica. Much quartz 

 was added, and generally also smaller amounts of potash, feldspar, biotite. 

 and sphene. Where little or no potash feldspar was added, as along the 

 northwest margin of the batholith, a quartz-rich diorite (tonalite) was 

 produced; where considerable amounts of potash feldspar were added, as 

 along the south and southwest margin, granodiorite formed. The inner 

 faeies, upon consolidation, was less calcic than the marginal, and origi- 

 nally ranged from a diorite to granodiorite, with oligoclase rather than 

 andesine. Postconsolidation emanations added considerable silica and pot- 

 ash and increased slightly the amount of biotite. Considerable added pot- 

 ash feldspar changed the rock to a quartz monzonite; less, to a grano- 

 diorite. Locally, enough potash was added to form granite; in places, a 

 muscovite granite. 



Younger Intrusives 



The great batholith is now known to be composite and to contain plu- 

 tons younger than the main mass of quartz monzonite or granodiorite. See 

 the Geologic Map of Idaho bv Boss and Forrester, 1947. Boss ( 1935) has 

 described an intrusion in the Casto district that cuts the Laramide struc- 



319 



