ABSTRACT Outcrops within the broad expanse of the Minnesota River Valley in southwestern Minnesota mark the southernmost exposures of the Archean Superior Province of the Canadian Shield. Despite their relatively restricted exposure, the Meso- to Paleoarchean gneisses in the Minnesota River Valley have received considerable attention due to both their antiquity and their complexity. The rocks exposed include the migmatitic Morton and Montevideo granitic gneisses, schistose to gneissic amphibolite, metagabbro, and paragneiss. The units have undergone upper amphibolite to granulite facies metamorphism, multiple periods of folding, and intrusion by a weakly foliated Neoarchean granitic unit (the Sacred Heart Granite) and Paleoproterozoic mafic dikes and adamellite granite. Classic geochronologic studies of the Minnesota River Valley gneiss terrane from the 1960s through the 1970s used K-Ar, Rb-Sr, and U-Pb zircon isotopic techniques to establish the antiquity of the gneisses and general aspects of the geologic history of the terrane. However, more recent U-Pb SHRIMP (sensitive high-resolution ion microprobe) zircon geochronology has considerably refined our understanding of the complex history of the gneiss terrane. These studies indicate that the oldest units in the Minnesota River Valley terrane crystallized ca. 3500 Ma, but the rocks subsequently saw new zircon growth associated with events at ca. 3440, 3385, 3140, and locally 3080 Ma. The Archean history of the terrane culminated with high-grade metamorphism ca. 2619 Ma and intrusion of the Sacred Heart Granite at 2604 Ma. In addition to visiting classic outcrops of the Morton and Montevideo Gneiss, this field trip includes stops at each of the major gneissic rock units in the Minnesota River Valley. We will examine field relationships that are the basis for both our general understanding of the deformation and metamorphic history of the gneiss terrane and the sampling strategies for our recent geochronologic and ongoing isotopic studies.

Abstract The glacial history of southern Minnesota, based on the surficial deposits of the late glacial Des Moines lobe and older deposits correlated to Marine Oxygen Isotope Stage 16, is documented in exposures along tributaries to the Minnesota River. A recently formalized stratigraphy defines and correlates these tills on the basis of their texture, lithologic composition, and stratigraphic position. Soils developed in this glacial landscape are among the most productive in the world. Their subtle variations reflect shifting ecotones throughout the Holocene.

ABSTRACT The Minnesota River Valley was carved by the draining of glacial Lake Agassiz ~13,400 years ago. Up to 85 m of incision along the proto–Minnesota River during this event spawned knickpoints that have been migrating upstream on tributaries, including the Le Sueur River in south-central Minnesota. This trip will explore the evolution of the Minnesota River Valley and Le Sueur River over the Holocene and discuss implications of landscape history for modern geomorphic processes. We begin with a brief tour of glacial stratigraphy and overview of the incised Minnesota River Valley. We then travel to the Le Sueur River to see bluffs, ravines, well-preserved strath terraces, and a paleo-channel that recorded incision and knickpoint migration of the Le Sueur River. We will discuss the process of landscape evolution in this otherwise low-gradient landscape and how the geologic history affects modern erosion and sediment loading to the Le Sueur, Minnesota, and upper Mississippi Rivers.

ABSTRACT This field trip will highlight landform sediment assemblages and the geomorphic consequences and timing of multiple significant deglacial flood events at and near the confluences of the Minnesota and St. Croix rivers with the Upper Mississippi Valley. This geographic position is also near the former margins of two different Late Wisconsin glacial ice fronts. New radiocarbon and optical spectral luminescence (OSL) ages collected from these landform sediment assemblages are presented to help date the geomorphically transformative Late Wisconsin and earliest Holocene flood events of this complicated river confluence setting. Trip discussions will include pre–Late Wisconsin bedrock valley fill; geomorphology and gradients of genetically related terraces in the Upper Mississippi Valley and major west-side tributaries; comparisons between radiocarbon and OSL dating results; GIS mapping technologies and data sets; and contexts and predictions for buried archaeological resources.

The lower Precambrian granulite-facies gneisses near Granite Falls and Montevideo have undergone four phases of folding, two periods of metamorphism, and both pretectonic and posttectonic intrusive events. Rare F 1 folds occur only in thin quartzofeldspathic veins that are at a high angle to the pervasive S 1 foliation and banding in the gneisses and that formed during the generation of S 1 . The F 2 folding generated the most prominent structural features in the area—a large-scale, gently plunging antiform near Granite Falls and an inferred synform between Granite Falls and Montevideo. The F 3 and F 4 fold phases are not evident from mapping of the major F 2 structures, but they are well represented by minor structures, and their relation to the F 2 folding event is deduced from an analysis of the minor folds and mineral lineations in the gneisses. This analysis indicates a systematic variation in the axial orientations of the minor F 3 and F 4 folds with their position on the limbs of the major F 2 structures. Analysis of the structures in a 3,050-m.y.-old granitic unit intruding the Montevideo Gneiss suggests the S 1 foliation and high-grade M 1 metamorphism accompanying S 1 were initiated prior to 3,050 m.y. ago. The timing of F 2 folding is not well constrained, but F 3 folding occurred after 3,050 m.y., and both events were accompanied by M 1 metamorphism. Mineral ages of 2,650 m.y. have been interpreted as resulting from the M 1 metamorphism and suggest an extreme duration from earlier than 3,050 m.y. to 2,650 m.y. for the high-grade metamorphism. The F 4 folding and intrusion of a set of tholeiitic diabase dikes followed M 1 metamorphism but preceded the formation of narrow shear zones about 2,400 m.y. ago. Intrusion of hornblende andesite dikes and a small quartz monzonite pluton 1,800 to 1,850 m.y. ago was accompanied by low-grade metamorphism (M 2 ) without deformation.

Geochemical and geochronologic data are presented for the Archean rocks in the Granite Falls area of the Minnesota River Valley, southwestern Minnesota. The rocks form two major groups: mafic and felsic gneisses. The mafic rocks include layered hornblende-pyroxene-plagioclase and biotite-pyroxene-plagioclase types with variants of each, amphibolite, and metagabbro. The biotite-pyroxene gneiss and some of the hornblende-pyroxene gneiss are metasedimentary with graywacke precursors. The amphibolites are of tholeiitic and basaltic komatiite composition, both of igneous derivation. Younger (Proterozoic) diabase dikes approach the tholeiitic amphibolite in composition. The felsic gneisses have a wide range of composition and include tonalite, granodiorite, adamellite, and pegmatitic granite, all of igneous derivation and of different ages. Tonalite, granodiorite, and lesser amounts of adamellite gneiss are interlayered on a large scale with the layered mafic gneiss, and the apparent concordant relationships may be, in part, the result of deformation, but may also be interpreted as (1) an original volcanogenic-sedimentary pile or (2) sill-like intrusions of felsic rocks in an older mafic basement complex. The older felsic and mafic gneisses were folded before the intrusion of younger granitic pegmatite and adamellite, and subsequently the region was folded in late Archean time. U-Pb analyses of zircon give minimum ages of 3,230 m.y. for the granodiorite and related old gneiss, 3,050 m.y. for the adamellite gneiss, 2,600 m.y. for the late Archean high-grade metamorphic event, and 1,800 m.y. for the Proterozoic igneous activity. Rb-Sr whole-rock analyses indicate an age of 3,600 m.y. for the older metamorphic complex and 3,000 to 3,100 m.y. for the pegmatitic granite and adamellite gneisses. Chemical and petrographic data reveal some of the interactions that occurred during the emplacement of the pegmatitic granite and adamellite and suggest that the magmas were emplaced approximately 3,050 m.y. ago. Mixed-rock and contaminated samples are not easily recognized in the field. These not only reflect interactions between country rock and the invading magmas, but also the high-grade metamorphism 2,600 m.y. ago and subsequent events that involved shearing and hydrothermal alteration. Although there are marked differences in rock types, metamorphic grade, and structure in the Granite Falls and Morton areas, the geologic history of the migmatitic terranes is similar. In the Granite Falls area the paleosome is largely granodiorite gneiss with minor amounts of adamellite and amphibolite, whereas in the Morton area it is tonalite gneiss and amphibolite with minor amounts of granodiorite. In both areas the neosome is pegmatitic granite and adamellite gneiss. The thick sequences of layered mafic gneiss at Granite Falls are lacking in the Morton area. The younger adamellite-2 and the aplite dikes, approximately 2,600 m.y. old, of the Morton area have not been found in the Granite Falls area.

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.

In the Minnesota River Valley, an epizonal, anorogenic granite that is often referred to as the granite of section 28 (lat 44°49.73′N, long 95°33.90′W) has been found to have a Pb-Pb age of 1.84 ± 0.05 b.y. on the basis of data obtained on HF leached and unleached feldspars and HCl leached and unleached whole rocks. The Th-Pb age of the feldspar-whole-rock pair is 1.9 b.y., which is in satisfactory agreement with the Pb-Pb age; but the U-Pb age is greater than the Pb-Pb age and probably indicates that uranium has been leached from the whole rock within the past several hundred million years, perhaps as a consequence of dilatancy resulting from uplift and erosion. Another granite, the mesozonal, late-tectonic Precambrian Sacred Heart Granite in the Minnesota River Valley (lat 44°41.3′N, long 95°21.5′W) is found to have a Pb-Pb age of 2.605 ± 0.006 b.y. on the basis of data obtained on HF leached and unleached feldspars and HCl leached and unleached whole rocks. Both the Th-Pb and U-Pb isochron ages are much older than the Pb-Pb age. An older age was expected for the U-Pb system as it had been previously found in the epizonal granite, but to also find it for the Th-Pb system was surprising. As is predicted for these kinds of granites in this type of tectonic environment on the basis of Mesozoic and Cenozoic analogues, the initial leads in the granites indicate that they were derived from source material having values of 238 U/ 204 Pb <9 normalized to the present day. This feature is common in Mesozoic and Cenozoic igneous rocks penetrating Precambrian terranes but is rarely observed in pre-Mesozoic igneous rocks. The Sacred Heart Granite is the oldest igneous rock known to show this effect and is the first representative of a mesozonal granite. The uranium depletion event appears to have been a granulite-facies metamorphism, but the age of that metamorphism cannot be determined from the available data. The model-lead-age information, however, suggests that it occurred before 2.78 b.y. ago. The source materials for both granites also underwent an earlier stage of extensive but unknown duration during which 238 U/ 204 Pb >9. In Phanerozoic rocks, such values are characteristic of ensialic tectonic environments. Similar development of ensialic environments was apparently occurring also in perancient times.