Abstract Precambrian rocks in the Lake Superior region underlie all or parts of Minnesota, Wisconsin, and Michigan, an area along the southern margin of the Superior province of the Canadian Shield (Fig. 1). Except on the north, adjacent to Canada, the Precambrian rocks are overlapped by sedimentary strata of Paleozoic and Mesozoic age, which constitute a thin platform cover of relatively undisturbed rocks that thicken to the west, south, and east. Inliers of Precambrian rocks are exposed locally in southern Minnesota and Wisconsin, mainly in the flat valleys of major rivers, where erosion has cut below the Phanerozoic strata. The present landscape is subdued, and is inherited largely from Pleistocene continental glaciations, which produced a variety of erosional and depositional landforms. The glacier ice scoured the bedrock in the northern parts of the region, in much the same way as throughout most of Canada, and deposited materials of diverse lithology and provenance, as much as 200 m thick, over much of the remainder of the region. The Precambrian rocks in the region record an extended interval of crustal development and evolution that spans nearly 3 b.y. of earth history. This interval of geologic time is not continuously recorded in layered and intrusive units, but instead is punctuated by specific rock-forming and tectonic events that can be deduced from geologic relations and placed in a chronometric framework by isotopic dating. (Fig. 2, also see correlation chart for Precambrian rocks of the Lake Superior region, Morey and Van Schmus, 1986; and Bergstrom and Morey, 1985.)

Abstract The Wyoming province is the region in Wyoming and adjacent states underlain by rocks of Archean age (Plate 2). It is an Archean craton bordered on the east and south by younger Precambrian provinces (Plate 1). Precambrian rocks are only exposed in the cores of the Laramide (Late Cretaceous to Early Tertiary) uplifts, and outcrops constitute less than 10 percent of the area underlain by Archean basement. Between uplifts, basement rocks are covered by thick Phanerozoic strata, so that extrapolations of geology from uplift to uplift are generally tenuous. Reconstructing the Archean history of the Wyoming province is further complicated by deformation associated with the late Mesozoic fold and thrust belt along the western margin. Although the Archean record is fragmentary, the basement uplifts generally afford excellent exposure. The northern and northwestern margins of the Wyoming province are poorly constrained. Archean rocks are known as far north as the Little Rocky Mountains of Montana (Peterman, 1981). According to King (1976), Archean and Early Proterozoic dates are intermingled at the northwest margin of the province in a wide zone that exhibits a gradational change from Archean dates in central Montana to Early Proterozoic dates to the northwest, reflecting increasing influence of post-Archean thermal events. The southwestern and southern margins of the Wyoming province are also poorly constrained. Archean rocks have been reported from several ranges in the Cordilleran orogenic belt, such as the Albion and Raft River Ranges (Armstrong and Hills, 1967) and possibly the Ruby Mountains of Nevada (A. W. Snoke, personal communication, 1986).

Two distinct granite plutons occur above the Main Central Thrust north of Uttarkashi in the upper valley of the Bhagirathi River and its source, the Gangotri Glacier, in Garhwal, India. The structurally lower pluton is a biotite granite with mineralogic and major and trace element characteristics similar to late Precambrian to early Paleozoic plutons of the Northern and Lesser Himalayan belts of Indian shield granites. The structurally higher pluton, intruded into the Martoli Formation and Vaikrita Group of the Tethyan sedimentary rocks, is an aluminous S-type muscovite-tourmaline leucogranite similar to other Cenozoic High Himalayan leucogranites with respect to its mineralogy and major element chemistry, as well as its high concentrations of Rb, Cs, and U, combined with low concentrations of Sr, Zr, Th, and rare-earth elements. Whole-rock Rb-Sr data for five samples from the main leucogranite define two clusters on an isochron of 64 ± 11 Ma. This isochron may reflect (1) a significant age, (2) variations of initial 87 Sr/ 86 Sr in the magma at the time of crystallization, or (3) multiple pulses of magma with different isotopic compositions. An Rb-Sr mineral isochron for one of the leucogranite samples yields an age of 21.1 ± 0.9 Ma, whereas a K-Ar age on a muscovite separate from the same rock yields an age of 18.9 ± 1.3 Ma. Whatever the actual age of the leucogranite magma within the range of these different determinations, it clearly had a high initial 87 Sr/ 86 Sr, greater than 0.746. The initial 143 Nd/ 144 Nd is correspondingly low, less than 0.51190. These data suggest that the granitic magma developed by anatectic melting of older continental crust. Although the timing of anatectic melting is equivocal, constraints on the conditions required to produce the melts imply that postcollisional, intracontinental subduction along the Main Central Thrust (MCT) could not produce the leucogranites without some preheating of the Indian continental crust. This heating may have occurred when delaminated Indian subcontinental lithosphere was replaced by hot asthenosphere in the initial stages of the continental collision. Crustal anatexis to produce the leucogranites might have already occurred at this early stage of collision. Alternatively, once the crust was so heated, crustal anatexis may have occurred within the hot crystalline Higher Himalayan slab due to influx of volatiles from the Lesser Himalayan slab as the former was emplaced over the latter by intracontinental subduction. Continued continental convergence led to uplift of the Higher Himalaya due to displacement along a major crustal footwall ramp of the Main Central Thrust. The mineral ages are interpreted as dating cooling related to this uplift, which may have coincided with termination of active motion of the MCT and the initiation of motion along the Main Boundary Thrust.

The Penokean foldbelt is a northeast-trending zone of deformed and metamorphosed Archean and early Proterozoic rocks, as much as 250 km wide in the Lake Superior region, along the southern margin of the Superior province. The rocks within the foldbelt were deformed 1,880–1,770 m.y. ago during the Penokean tectonothermal event. Evolution of the foldbelt began with rift-faulting that was localized along a preexisting zone of weakness in Archean rocks, the boundary between the two crustal segments previously recognized in the region: a greenstone-granite terrane (~2,700 m.y. old) to the north (Superior province) and a mostly older (in part 3,500 m.y. old) gneiss terrane to the south. The faulting and later broad foundering provided sites for deposition of detritus shed mainly from the craton to the north and for chemical sediments, including the vast iron-formations for which the region is famed. The central part of the basin also received mafic volcanic rocks intimately intercalated with the sedimentary rocks, and similar rocks dominate in the southern part. The terminal, compressional stage ended deposition. It involved folding of the supracrustal rocks, penetrative deformation of the basement rocks, and emplacement of diapiric gneiss domes with accompanying appression of intervening supracrustal rocks. Deformation was complex and prolonged, and because of basement participation it differed from place to place in style and orientation of folds and in intensity and nature of metamorphism. Granite-tonalite plutons locally were emplaced in the southern part of the foldbelt near and after the end of deformation. The rifting and terminal compression occurred in an intracratonic or continental-margin environment; possibly the compression was caused by forces transmitted from a remote distance to the southeast, in an area undergoing rifting of a continental margin and subsequent continent-continent collision.

The Marenisco-Watersmeet area in the western part of the northern peninsula of Michigan contains a greenstone and granite terrane (Puritan Quartz Monzonite) of late Archean age on the north and a gneiss terrane (gneiss at Watersmeet) on the south. A granite and gneiss belt (collectively called the granite near Thayer) crops out between these contrasting terranes. Lower Proterozoic metasedimentary and metavolcanic rocks of the Marquette Range Supergroup are extensive. Radiometric dating of the tonalitic phase of the gneiss at Watersmeet establishes an early Archean age and a complex subsequent history. U-Th-Pb systematics provide a firm minimum age of 3,410 m.y. with the possibility of a much greater age—3,500 to 3,800 m.y. Cataclasis and recrystallization during the early Proterozoic Penokean orogeny are recorded on a regional scale by whole-rock and mineral Rb-Sr ages of 1,750 m.y. Intense cataclasis of granodioritic gneiss in the Watersmeet dome locally produced metamorphic zircon with concordant ages of 1,755 m.y. Zircons from a tonalitic phase of the granite near Thayer are dated at 2,750 m.y. Zircons from leucogranite dikes, which are abundant in the tonalitic phase of the gneiss at Watersmeet, are slightly younger at 2,600 m.y. These intrusive rocks are approximately contemporaneous with the development of the greenstone-granite terrane of late Archean age.

Granitic rocks ranging in composition from granite to tonalite and associated metavolcanic-metasedimentary rocks compose an east-trending belt as much as 180 km wide and 300 km long in northern Wisconsin. The granitic rocks have an initial 87 Sr/ 86 Sr of 0.7025 ± 0.0005 and a Rb-Sr whole-rock isochron age of 1,885 ± 65 m.y., which is interpreted as the time of crystallization of the granitic rocks. Rb-Sr whole-rock mineral secondary isochrons for two samples give ages of 1,655 ± 55 m.y. and 1,545 ± 55 m.y.; K-Ar ages of biotite from these samples are 1,615 ± 55 m.y. and 1,598 ± 54 m.y., respectively. These mineral ages are interpreted as resulting from isotopic resetting caused by a thermal event about 1,600 m.y. ago. The granitic rocks and associated metavolcanic-metasedimentary rocks constitute lower Proterozoic greenstone-granite complexes that are remarkably similar in pattern to the Archean greenstone-granite complexes in the Superior province of the Canadian Shield.

The Rainy Lake region on the boundary between Ontario and Minnesota is a classical Precambrian area in which two periods of folding and igneous activity, Laurentian and Algoman, were recognized by Lawson. Early efforts to resolve the two orogenic periods on the basis of K-Ar and Rb-Sr age determinations on micas were not successful. More sophisticated whole-rock Rb-Sr and zircon and sphene U-Pb studies likewise have not been wholly successful, but the U-Pb data suggest that all the early Precambrian events, including the Laurentian and Algoman igneous activity, probably occurred within the interval from 2,700 to 2,750 m.y. ago. Whole-rock Rb-Sr isochron studies, however, give younger ages as follows: Coutchiching Series 2,615 ± 50 m.y. Keewatin Series 2,595 ± 45 m.y. Algoman granites Ontario 2,540 ± 90 m.y. Minnesota 2,680 ± 95 m.y. Mineral ages and a rock-mineral isochron clearly indicate that both the Rb-Sr and the K-Ar systems were affected by subsequent events, but it is not certain that the isochron ages date the time of specific events. It is more likely that the determined ages mark the end of periods of retrograde metamorphism, faulting, and shearing. The different apparent ages of stabilization of the Rb-Sr system probably resulted from differences in rock composition, in water content, and in local heat flow.