320 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



tures and perhaps even Miocene (?) beds. It consists of a pink granite 

 to a quartz monzonite. The map of Fig. 21.2 shows this intrusion, as well 

 as others of similar age and relation to the main batholith. The data were 

 taken from the Tectonic Map of the United States. Ross also mentions 

 pink granites in the northwest corner of Idaho and in Rritish Columbia 

 that probably cut Miocene (?) strata, and which he believes are distinctly 

 younger than the Nelson batholith. These pink granites lead the writer 

 to think of a pink granite in southwestern Montana which proved to be 

 Precambrian in a thrust sheet which was later cut and displaced against 

 Miocene ( ? ) basin beds by a high-angle fault, as if in intrusive contact 

 with them. 



Anderson ( 1948 ) describes two areas of younger intrusives within 

 the main batholith and says there are "many others." The younger in- 

 trusives are of two sets, one believed to have been emplaced at the close 

 of the Laramide orogeny and the other in mid-Tertiary time. The early 

 Tertiary magma was chiefly noritic, and the mid-Tertiary injections range 

 in composition from dacite to rhyolite, with quartz monzonite porphyry 

 and rhyolite porphyry most abundant. The Tertiary plutons within the 

 main batholith are small and elongated. One, however, Anderson de- 

 scribes as 8 miles long and % to 1% miles wide. They invade fault and 

 shear zones, the main ones of which extend in a northeast direction. 



Again in 1952 Anderson cites evidence that discrete masses of the 

 granitic rock were emplaced under deep-seated conditions and others at 

 much shallower depths. The deep-seated plutons include one that evolved 

 while the major orogeny was taking place, and another which came in 

 during the later, less intense stages of deformation. The shallower intru- 

 sions are those of Laramide and later age. 



Border Zones 



The Thatuna pluton, a satellite on the west, is principally granodiorite 

 but grades into adamellite, tonalite, and granite. The Beltian strata which 

 the Thatuna batholith intrudes are variably affected. In extremely fine- 

 grained types, the contact is sharp and follows joint planes; but the con- 

 tact with the granular quartzite is gradational through several hundred 

 yards. By increase in feldspar the quartzite grades into igneous rock. To 



the southeast of the Thatuna pluton, thin layers of pegmatite and aplite 

 are interlayered with paragneiss and diopside quartzite to form an exten- 

 sive mass of gneiss. The belt of gneiss is 12 miles wide in Latah County, 

 and extends for 15 miles at least into Clearwater County, where it borders 

 the Idaho batholith. An extension of the Idaho batholith is believed to 

 underlie the metamorphic belt (Tullis, 1944). 



The Bitterrot Range of Idaho and Montana is largely a zone of gneiss 

 and schist that borders the Idaho batholith on the northeast corner. It is 

 a migmatite of the intrusion, according to Langton ( 1935 ) , but according 

 to Sydney Groff of the Montana Bureau of Mines and Geology (personal 

 communication) it is a Precambrian terrane. 



AGE 



Consanguinity 



The age of the great Idaho batholith is an important problem in the 

 tectonic setting and, at the same time, a matter of controversy. The prob- 

 lem seems to be resolved into an issue between a Nevadan and a Lara- 

 mide age. 



The principal argument advanced for the Nevadan age of the Idaho 

 batholith is its lithologic similarity to the batholiths of the Nevadan orog- 

 eny, specifically to the Nelson batholith (Ross, 1928). Since all the batho- 

 liths exhibit many variations in the granitoid series, generally, from diorite 

 to granite, it does not seem possible to correlate them closely in age on the 

 basis of lithologic similarity. It must be granted, however, that the granit- 

 oid character, together with great size and clustered grouping, seems to 

 relate them to a common great orogenic belt and batholithic cycle. Noth- 

 ing similar to the Idaho batholith occurs elsewhere in the Laramide oro- 

 genic belt. 



Intrusive Relations 



Near its southeastern end the batholith intrudes a thick series of Paleo- 

 zoic strata. In this vicinity, isolated granitic masses similar and probably 

 satellitic to the batholith cut Paleozoic strata as young as Pennsylvanian. 

 Farther north the bordering formations are mostly Proterozoic ( Beltian ) 



