The Morton Gneiss of southwestern Minnesota is a migmatitic hybrid rock. The paleosome consists of tonalitic to granodioritic gneisses and amphibolites; the neosome, of a variety of granitic gneisses. The older rocks are largely biotite-quartz-oligoclase gneisses with minor microcline and hornblende and are tonalitic in composition grading to granodiorite. Locally, shearing and recrystallization of layered tonalitic gneiss developed a granoblastic granodioritic gneiss with idioblastic crystals of hornblende in a granular matrix of microcline, quartz, and oligoclase. Amphibolite is closely related to the tonalitic gneisses and occurs as pieces or clasts that range in size from small pods or schlieren to large angular blocks. Some of the amphibolite is younger than the tonalitic gneisses and represents fragmented dikes or sill-like masses; however, some may be of the same age or older than the tonalitic gneisses. Occurrence and mineralogy indicate that the amphibolites are all of igneous precursors with compositional variations suggestive of tholeiitic to basaltic komatiite parentage. The neosome is composed of granitic varieties from oldest to youngest: (1) pegmatite and microcline granite, (2) fine-grained adamellite (adamellite-1), (3) granodiorite and (4) microadamellite porphyry at Morton and gneissic fine-grained adamellite to the northwest (adamellite-2). At least two major periods of deformation are recognized. The older involved the tonalitic gneisses and the amphibolites and was developed before the emplacement of the granitic phases. The tonalitic gneiss-amphibolite complex was disrupted at the time of the intrusion of the pegmatitic and adamellitic magmas, but it is not clear how much of the present-day structure of the Morton Gneiss was developed at that time. The second major deformation affected the granitic phases as well as the older tonalitic gneiss-amphibolite complex and was responsible for the highly contorted structure of the Morton Gneiss. The rocks included in adamellite-2 are least deformed, and these appear to have been emplaced in the late stages of the second major deformation. Straight, relatively undeformed dikes of aplite, aplite-pegmatite, and pegmatite cut the Morton Gneiss. They represent the last Archean igneous activity in the region. A poorly developed foliation in the aplite dikes is attributed to stresses generated during the uplift of the region. In the southeastern part of the Morton area, between Franklin and New Ulm, there are Proterozoic diabase dikes and small plutons containing granitic phases. These rocks were intruded approximately 1,800 m.y. ago.

The paleosome of the hybrid Morton Gneiss consists of tonalitic and granodioritic gneisses with enclaves or clasts of amphibolite. The neosome contains granitic gneisses ranging from granodiorite to K-rich granite. The major rock groups are differentiated in plots of the normative orthoclase:plagioclase: quartz ratios and of K 2 O versus Na 2 O. These groups are further subdivided on the basis of major, minor, and trace elements in discrimination diagrams in which both concentrations and ratios of elements are considered. All the principal rock types were originally of igneous origin. The tonalite gneisses are subdivided into Fe-rich and Fe-poor varieties; both are biotite-quartz-oligoclase assemblages with minor microcline and hornblende and are classed as trondhjemitic gneiss. With increase in microcline, the tonalite gneiss grades to granodiorite gneiss that cannot be differentiated in the field but is a distinct petrographic and chemical type. Locally, shearing and recrystallization of layered tonalite gneiss formed veins of granoblastic granodioritic gneiss. Relatively large amounts of Rb were lost during the shearing and recrystallization. The basic chemical patterns of the amphibolites indicate that they were derived from basaltic precursors of two different types. A hornblende-plagioclase amphibolite is tholeiitic, and a hornblende-rich, low-alumina variety is komatiitic in composition. Both are Fe-rich, and within each group there are variations, which are assigned to magmatic processes, but also some that are related to secondary metasomatic processes. Relatively large amounts of K and Rb (and, in some samples, also Ba and Sr) were introduced. The amphibolite also must be regarded as the source of some contamination by physical mixing during deformation of the closely associated tonalite gneiss. Microcline pegmatitic granite, granodiorite, and adamellite are recognized in the granitic gneiss phase of the Morton Gneiss. Two groups of adamellite are distinguished: an older gneiss called adamellite-1 and a younger gneiss called adamellite-2. Adamellite-2 is further subdivided into microadamellite porphyry that occurs at Morton and a gneissic fine-grained adamellite at localities northwest of Morton. The microadamellite porphyry at Morton chemically resembles a phase of the Sacred Heart Granite, but the gneissic adamellite to the northwest resembles in composition some of the aplite dikes that traverse the structure of the Morton Gneiss. The aplites, however, show the effects of strong fractionation, and the average aplite composition differs from that of adamellite-2 in the relative enrichment in Rb and impoverishment in Sr and Ba.

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 207 Pb- 206 Pb 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 87 Sr. 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 87 Sr/ 86 Sr ratio, and the veinlets were formed by hydrothermal activity probably associated with the 1,800-m.y. B.P. event. Redistribution of radiogenic 87 Sr 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.

Two major mineralogic types of mafic enclaves (amphibolite and pyroxene-amphibolite) have been characterized in the Morton quartzofeldspathic gneiss at Morton, Minnesota. Some amphibolite contains plagioclase megacrysts with An 66 cores and An 43 rims. One indication of compositional equilibration of the amphibolite with host gneiss is the presence of biotite-rich rims on many enclaves. Textural and compositional reaction relationships observed in the mafic enclaves may be interpreted as relict igneous features. Bulk compositional data from three representative enclaves at Morton suggest that the pyroxene amphibolite could be derived from amphibolite by crystal fractionation and that both enclave types could have crystallized from a tholeiitic magma.