The Morton Gneiss in the Minnesota River Valley, southwestern Minnesota, consists of an older metamorphic complex of tonalitic gneisses and amphibolites (the paleosome) and younger granitic gneisses (the neosome). Discordant U-Pb ages on zircon from the tonalitic gneisses give a minimum age of 3,300 m.y. with indications of an older age. An age of approximately 3,500 m.y. is indicated by Rb-Sr data for the tonalitic and related granodioritic gneisses; however, subsequent events have disturbed the Rb-Sr as well as the U-Pb systems in the Archean rocks. In addition to the early folding that formed the metamorphic complex, these subsequent events include high-grade metamorphic events 3,050 and 2,600 m.y. ago and a thermal event in Proterozoic time, 1,800 m.y. ago.

The Rb-Sr isotopic system has been variously affected in different rock units as a result of loss and gain of both Rb and Sr. Rb loss from veins of granoblastic granodioritic gneiss formed by shearing and recrystallization of layered tonalitic gneiss results in ages that are too old. Secondary isochrons for the amphibolites and for the tonalitic and granodioritic gneisses in the Morton vicinity reflect Rb gain approximately 3,000 m.y. ago. The geologic relationships suggest that the Rb, K, and other elements were introduced at the time of invasion of the tonalitic gneiss-amphibolite complex by the pegmatitic granite and adamellite-1 magmas. The average ²⁰⁷Pb-²⁰⁶Pb age of four samples of zircon from the pegmatitic granite gneiss is 3,043 ± 26 m.y., but because of discordance in the U-Pb ages, a somewhat older original age cannot be precluded.

The pegmatitic granite gneiss, adamellite-1 gneiss, and agmatic granodiorite, the principal units of the neosome, all give Rb-Sr ages that are too young because of loss of radiogenic ⁸⁷Sr. Loss of radiogenic Sr from some samples of the aplite dikes is clearly shown in model ages that range from 2,060 to 2,465 m.y.; whereas the Rb-Sr isochron age of less-disturbed samples is 2,590 ± 40 m.y. [R_i , 0.7036 ± 0.0020 (95% C.L.]. Samples of adamellite-2, which is chemically similar to the Sacred Heart Granite (2,600 m.y.) and to the aplite dike rocks, give an isochron age of 2,555 ± 55 m.y. (R_i , 0.7029 ± 0.0013). These rocks probably were emplaced in a late stage of the major deformation 2,600 m.y. ago that affected both the neosome and paleosome and developed the structure of the Morton Gneiss. Veinlets of well-crystallized epidote are common in adamellite-2 and in older rock units. The Rb-poor, Sr-rich epidote has a high ⁸⁷Sr/⁸⁶Sr ratio, and the veinlets were formed by hydrothermal activity probably associated with the 1,800-m.y. B.P. event.

Redistribution of radiogenic ⁸⁷Sr among the mineral phases and between intimately mixed rock units has occurred in the Morton Gneiss—hence the need for careful sampling and interpretation of the Rb-Sr results. A relatively good isochron [2,640 ± 115 m.y.; R_i , 0.7048 ± 0.0014 (95% C.L.] for samples of adamellite-1 and the secondary isochrons for the amphibolites and the tonalitic and granodioritic gneisses (about 3,000 m.y.) are essentially mixing lines.