PRECAMBRIAN TECTONIC PROVINCES 





ably on the crystalline basement, and are variously called Algonkian, 

 Proterozoic, Beltian, or Upper Precambrian. These are shown on the map 

 iby the dotted lines. Extending southwestward from the western part 

 of Lake Superior is another belt of late Precambrian rocks, namely the 

 Keweenawan Series with its included large gabbro sills. Beneath the 

 Paleozoic and Mesozoic sedimentary cover of Texas and southeastern 

 New Mexico still other young Precambrian sediments, volcanics, and 

 gabbro sheets have been recognized, resting on an older granitic terrane. 



Algoman Oogenic Belt 



The ages thus far published for north-central Wyoming and south- 

 central Montana are very old (2500 to 2760 m.y.) and stand apart from 

 other ages in the Rocky Mountains (Aldrich et al., 1957; Gast and Long, 

 1957; Hayden and Wehrenberg, 1959 ) . An absolute age determination in 

 southeastern Manitoba between Winnipeg River and Johnston Lake indi- 

 cates that a plutonic and metamorphic cycle occurred 2650 =■= 100 m.y. 

 ago (Eckelmann and Gast, 1957). These ages are 400 to 500 m.y. older 

 than those recorded for the "Superior" Province in Canada, but even so 

 iare much closer to it than to those of the adjacent younger orogenic 

 ; belt, and hence are regarded related. 



Penokean Orogenic Belt 



A number of isotope ages to date seem to establish an orogenic belt 

 ;of intermediate age between the very old Algoman and the younger 

 Mazatzal. These are in the range of 1600 to 1750 m.y. See Fig. 4.3. The 

 belt contains a mixture of the old dates and the younger, and this is 

 taken to mean that the younger orogeny was superposed on the older. The 

 analyses are so few to date that the northern limit of the belt is poorly 

 defined, and not much reliance for tectonic interpretive purposes can 

 yet be placed on the distribution. The southern limit is somewhat better 

 defined, with none of the older dates in the general field of the 1250 

 to 1450 m.y. dates. 



On the basis of the geology of the rocks of southwestern Montana the 

 two ages are understandable. Perhaps even more ages within the belt will 

 be recognized. A brief description of the recognized units is as follows: 



(1) The oldest units underlie the Cherry Creek Group and Include 

 of the Poiin' Group as well as other pre-Cherry Creek rocks which probably 

 an- not time equivalents ol the Pony. Main types present are bioriti 

 gneiss, granite gneiss, injection gneiss, and amphibole gneiss. 1 The CI* 

 Creek Group consists of mctascdiincnts including marble, quaitzite, micai i 

 schists, sillimanitc schist, handed ironstones with Intercalated layers of 

 amphibole gneiss, and amphibolite representing metamorphosed mafic silk and 

 flows. (3) A number of post-Cherry Creek intrusives, all of which show - 

 ing degrees of metamorphism, include, among others, the Dillon granite gn< 

 widespread in Beaverhead and Madison counties, the granite of the [ardine 

 district, and the Pinto metadiorite in the Little licit Mountains. (4) Wider) 

 distributed bodies of unmetamorphosed peridotite and associated ultrainafu 

 rocks have as their largest representatives the Stillwater Complex. (5) Post- 

 Stillwater intrusives are represented mainly by the granite of the Beartooth 

 Range. (6) Numerous and widespread diabase dikes that cut all these older 

 units but do not extend into Beltian rocks (Ileinrich, 1953). 



The crystalline basement of the Beartooth Range from what is known 

 consists of schists and gneisses, possiblv the Cherrv (.'reck. On the north- 

 east is the Stillwater ultramafic complex which has been intruded into a 

 series of dense gray hornfels, an iron formation, and light-colored quartz- 

 ites. It may be part of the Cherrv Creek group. A light gray gneissoid 

 biotite granite cuts the ultramafic complex. At Cook City two granites are 

 recognized (Parsons and Bryden, 1952). 



The roof of a granitic batholith is exposed in the Teton Range of 

 western Wyoming. The deep canyons that dissect the range show gigantic 

 zenoliths and an irregular roof of gneiss and schist. 



Mazatzal Orogenic Belt 



Distribution of Dates. A good scatter of age determinations has been 

 made in the Rockies from the Black Hills to Arizona and southern 

 Nevada and defines a belt of rather consistent age between 1300 and 

 1400 m.y. old. A low age is given for the Front Range of central Colorado 

 of 1100 m.y., a high age for the Black Hills of South Dakota of 1600 m.v., 

 and a high age of 1590 m.y. for the Central Wasatch Mountains in Utah, 

 Other than these three, ten other ages fall fairly close to 1350 m.y. 



No orogenic belt or province in the Canadian Shield has yielded such 

 ages. The 1350-m.y.-old belt of the western United States appears to 



