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ABSTRACT

The goal of this field excursion is to provide an opportunity for focused exploration of field-based teaching and learning. Learning in the field is an essential element in the education and development of geology students, but until recently has received relatively little attention by education researchers. Set against the backdrop of classic bedrock localities of the Midcontinent Rift System in northeastern Minnesota and using new knowledge of how people learn, of learners, and of pedagogical practices, this excursion explores goals for student learning in the field and effective instructional practices for helping students realize those goals.

Introduction

The purpose of this excursion is to create opportunities for educators to share knowledge, experiences, and resources for teaching in the field. Using some classic bedrock outcrops of northern Minnesota as backdrops, we will discuss a wide range of topics related to teaching in the field, including logistics, field guides, active learning pedagogies, the assessment of learning, using technology in the field, and perhaps even a few “teaching-in-the-field catastrophes.”

In preparing this guide, we considered several different designs and audiences (e.g., geologists, instructors, and students). In the end, we decided that all of us are first-and-foremost geologists and that whatever our goal, we enjoy seeing and learning about rocks in the field. With this in mind, the guide has a traditional structure and emphasis on geology. We also hope that teaching and student learning in the field will be an important topic of conversation “on the outcrop” and have added callouts for each stop that focus on “Opportunities for Student Learning.” Had we written the guide solely for our students, it would not contain as many stops, nor would it include many of the details that we hope they will discover through observation and discussion. Instead, this guide emphasizes geological information and pedagogical considerations that will likely be relevant to instructors for planning instructional activities at these stops. Participants on the trip will have opportunities to experience all three roles as geologists, instructors, and students. We have also tried to model this guide and the field trip using many of the best practices (e.g., learning goals, opportunities for active learning and knowledge construction, and assessments) described by Mogk (this volume) and in the pedagogical guide for introductory courses (Pound et al., this volume).

In the past few decades, many people have noted the need for changes in the way we teach (e.g., Wiggins and McTighe, 1998; Bransford et al., 2000; Ambrose et al., 2010), and much attention has been focused specifically on improving teaching and learning in STEM classrooms (e.g., American Association for the Advancement of Science, 1989; National Research Council, 2005; Handelsman et al., 2007). Recently, some researchers have also extended this call for change to field education (e.g., Manduca and Carpenter, 2006; Boyle et al., 2007; Maskall and Stokes, 2008; Whitmeyer et al., 2009). Geology is inherently a field-based science. Therefore, a field setting provides an opportunity to bring richer meaning to the content and skills learned in the classroom and laboratory. Learning in the natural environment also poses many new challenges for students, and the link between cognition and affect is complex (Fig. 1; Stokes and Boyle, 2009). Among other things, students must manage ambiguity and the complexity of outcrops compared with “specimens-in-a-box”; think spatially; apply new techniques and skills; adjust to the natural environment; work autonomously and in teams; and control (self-regulate) their motivations, attitudes, and beliefs. A brief introduction to research on learning in the field, and a growing list of resources for teaching in the field can also be found on the Science Education Resource Center website: http://serc.carleton.edu/research_on_learning/synthesis/field.html.

Many of us were lured to geology by our early field experiences, by the promise of a lifelong connection with nature, or by the draw of working outdoors. Field trips helped shape the values, beliefs, attitudes, and goals that excited us and motivated us to learn. How can we better help students clarify and develop their own professional and personal values and attitudes about the natural world and their ability to understand it? Do current (Millennial) and future generations of students come to the field with different values, beliefs, and attitudes, and do they respond to the challenges of the field environment in the same ways that we do? How might we modify traditional field instructional practices so that we attract and retain more students from underrepresented groups? These questions can help us address student learning on several levels beyond just a command of the geology.

Regional Geology

A comprehensive review of the literature on the geology of Minnesota is beyond the scope of this guide. In writing this guide, we drew heavily from several widely available and excellent guides and publications on the geology of northern Minnesota, including those by: Miller (1995a), Boerboom et al. (2004), Jirsa et al., (2004), Miller et al. (2002a), and Peterson et al. (2009). Besides the references listed here, both the Institute on Lake Superior Geology (http://www.lakesuperiorgeology .org/publications/proceedings.html) and the Minnesota Geological Survey (http://www.mngs.umn.edu/mgsdocs.html) have published excellent and useful guides to the geology of the northern Minnesota.

Figure 1.

A design model for learning activities for fieldwork. From Stokes and Boyle (2009).

Figure 1.

A design model for learning activities for fieldwork. From Stokes and Boyle (2009).

Basement Rocks of Northern Minnesota

Although this trip focuses on the geology of the Midcontinent Rift (Figs. 2, 3, and 4), Archean and Paleoproterozoic basement rocks played an important role in the origin and evolution of the rift. Below is a summary of the Archean and Paleoproterozoic geology from Peterson and Severson (2002) and Ojakangas et al. (2005).

The Archean terranes of northern Minnesota (Figs. 2 and 3) are the western extensions of the Wabigoon, Quetico, and Wawa subprovinces of the Superior Province (Card, 1990). The Wabigoon and Wawa subprovinces are composed largely of metavolcanic supracrustal assemblages and contemporaneous granitoid intrusives that are typical of granite-greenstone terranes. The Quetico subprovince, an intervening belt of mostly high-grade metasedimentary rocks, separates these two volcano-plutonic subprovinces. The Wawa subprovince is basement to the Mid-continent Rift and comprises a typical greenstone belt with thick sequences of volcanic rocks (Ely Greenstone assemblage; ca. 2.72 Ga) and a medial Algoma-type banded iron formation (Soudan Iron Formation). The lower part the Ely Greenstone is dominantly tholeiitic to calc-alkaline basalt, but also includes andesite, dacite, and rhyolite. Field and geochemical evidence suggest formation in a subsiding suprasubduction zone environment. Above the Soudan Iron Formation, the volcanics are dominantly tholeiitic basalt, believed to have formed by shield volcanism at greater water depths. Thick sequences of dacitic volcaniclastic rocks and turbiditic greywacke and slate (Lake Vermilion Formation) overlie the metavolcanic rocks. Continuing up in the section, these are successively overlain by greywacke, slate, and conglomerate (Knife Lake Group), and by mafic volcanics and ultramafic intrusives (Newton Lake Formation). The Ely Greenstone has recently been the target of exploration for volcanogenic hosted massive sulfide deposits and gold mineralization (Peterson et al., 2009). The supracrustal rocks are variably deformed and intruded by ca. 2.67–2.69 Ga granitoids (e.g., Vermilion Granitic Complex and Giants Range batholith). A swarm of radiating, NW-trending mafic dikes is prominent on aeromagnetic maps of Minnesota (Fig. 1). These Paleoproterozoic (ca. 2.07 Ga) dikes are composed largely of tholeiitic basalt from a depleted mantle source, minimally contaminated by continental crust or lithospheric mantle (Wirth and Vervoort, 1995).

Sediments of the Paleoproterozoic Animikie Group, deposited in a migrating foredeep during Penokean (1900–1760 Ma) folding and thrusting associated with south-directed subduction lie on top of the Archean supracrustal rocks (Southwick et al., 1988). In the northern part of the Animikie Basin, terrigenous clastic sediments of the Pokegama Formation were deposited on an Archean peneplain (Ojakangas et al., 2005). Detrital zircons from the Pokegama Formation were largely derived from ca. 2.7 Ga crust, likely the nearby granite-greenstone terranes to the north (Wirth et al., 2006). The next younger unit in the northern part of the basin is the Biwabik Iron Formation, a series of cherty and slaty Superior-type iron formations deposited on the foreland bulge of the Animikie Basin. Zircons from reworked volcaniclastic rocks in the upper Gunflint Iron Formation (the northern equivalent of the Biwabik) yielded a U-Pb age of 1878 Ma (Fralick et al., 2002). The widely distributed ca. 1850 Ma Sudbury ejecta deposit is also recognized near the top of the Gunflint Iron Formation. In the southern part of the Animikie Basin, the Virginia Formation and its southern equivalent, the Thomson Formation, consist of argillite, siltstone, and greywacke that came from sources in the south and were deposited as a series of turbidite successions. U-Pb ages of detrital zircons in the Thomson Formation are dominantly 2000–1800 Ma, reflecting derivation from the juvenile Penokean island arc to the south (Wirth et al., 2006). Deformation within the Virginia and Thomson formations increases to the south, and metamorphism within the Biwabik and Virginia formations increases eastward toward the contact with the basal Duluth Complex, locally reaching pyroxene hornfels facies. Younger thermal and tectonic features that overprint the Penokean orogen are now attributed to the geon 17 Yavapai Orogeny (Holm et al., 2007).

Figure 2.

Superimposed magnetic on gravity (SMOG) map of Minnesota showing first vertical derivative of aero-magnetic data and raw gravity data for Minnesota. The derivative transformed magnetic data (shaded in gray) show relative magnetic susceptibility (lighter shades are more magnetic) and enhance the signatures of the near-surface bedrock, whereas the untransformed gravity data emphasizes sources (red shades are more dense; blue shades are less dense) deeper in the crust. Prepared by Val Chandler (Minnesota Geological Survey). Terrane boundaries from NICE Working Group (2007).

Figure 2.

Superimposed magnetic on gravity (SMOG) map of Minnesota showing first vertical derivative of aero-magnetic data and raw gravity data for Minnesota. The derivative transformed magnetic data (shaded in gray) show relative magnetic susceptibility (lighter shades are more magnetic) and enhance the signatures of the near-surface bedrock, whereas the untransformed gravity data emphasizes sources (red shades are more dense; blue shades are less dense) deeper in the crust. Prepared by Val Chandler (Minnesota Geological Survey). Terrane boundaries from NICE Working Group (2007).

Figure 3.

Generalized geologic map of Minnesota (after Morey and Meints, 2000).

Figure 3.

Generalized geologic map of Minnesota (after Morey and Meints, 2000).

Figure 4.

Generalized geologic map of the northeastern Minnesota. Modified from Miller et al. (2001).

Figure 4.

Generalized geologic map of the northeastern Minnesota. Modified from Miller et al. (2001).

Midcontinent Rift System

The Midcontinent Rift System is one of the best-exposed rift systems in the world and offers unparalleled opportunities to study both intrusive and extrusive products and processes of continental rifting. This 2500-km-long tectonic feature extends northwest from the Grenville Front near western Lake Erie, in a large arc through Lake Superior, and then southwest to Kansas. Covered by Paleozoic sedimentary sequences along much of its length, the rift trend is easily traced across the midcontinent region (Fig. 2) by its distinctive aeromagnetic and gravity anomalies (Chandler, 2002). Very extensive exposures of the rift system are found along the North Shore of Lake Superior, where flood basalt, comagmatic intrusive rock, and late sedimentary sequences together are more than 13 km thick.

Miller et al. (2002b) provide an excellent review of past geologic mapping and mineral exploration in the northeastern Minnesota segment of the Midcontinent Rift, and Severson et al. (2002) describe the mineral potential. Recent mapping in northeast Minnesota culminated in the publication of a new map of the northwest limb of the Midcontinent Rift (Miller et al., 2001, 2002c) that formalizes much terminology and unit names. The rift-related rocks exposed in northeastern Minnesota, underlain by Archean and Paleoproterozoic rocks to the west (Figs. 3 and 4), dip generally east to southeast. The volcanic sequences North Shore Volcanic Group; Green, 2002) exposed along the north shore of Lake Superior (Fig. 4) are divided into two basins referred to as Southwest Limb and Northeast Limb (Green, 2002). The earliest (reversely polarized) flows in the Southwest Limb, termed the Southwest Lower Sequence, are exposed southwest of Duluth. Successively younger flows northeast of Duluth are younger (normally polarized) and termed the Southwest Upper Sequence. A similar volcanic sequence is exposed in the Northeast Limb, but flows in that basin are younger to the southwest along the lakeshore. The North Shore Volcanic Group is largely bimodal, consisting of tholeiitic basalt, andesite, icelandite, and rhyolite (Fig. 5), with felsic rocks composing up to 20% of the NE Limb.

Extensive co-magmatic, shallow-level dikes, sills, and sheet-like intrusions (e.g., Beaver Bay Complex) intruded the North Shore Volcanic Group during the Main Magmatic Stage (Miller and Green, 2002). Most of these relatively thin mafic to felsic intrusions intruded at medial to shallow levels within the rift succession (Fig. 4). Subvolcanic intrusions belonging to the Duluth Complex are found deeper in the rift system (Miller and Severson, 2002). These differ from the hypabyssal intrusions (e.g., Beaver Bay Complex) in that they more typically display evidence (e.g., igneous layering, cumulate textures, mineral foliation) of in situ differentiation and they have contacts with other plutonic rocks (Miller and Severson, 2002). The Duluth Complex is divided into four series (Miller and Severson, 2002), consisting of: (1) Felsic Series (mostly granophyre); (2) Early Gabbro Series (layered gabbro); (3) Anorthositic Series (unlayered plagioclase-rich gabbroic cumulates); and (4) Layered Series (layered troctolitic to ferrogabbroic cumulates). Rocks of the Felsic Series and Early Gabbro Series formed during the Early Magmatic Stage and occur mostly along the western and northern margin of the rift near the base of the Duluth Complex. Anorthositic Series and Layered Series rocks formed primarily during the Main Magmatic Stage.

Geochemical and geochronological evidence from Minnesota, Wisconsin, Ontario, and Michigan indicate that most rift-related magmatism occurred within a 23 million year time span at ca. 1100 Ma and suggest broad trends in magmatic sources and processes (Shirey et al., 1994; Miller et al., 1995; Miller and Vervoort, 1996; Nicholson et al., 1997). During the early stages of rifting (1109–1107 Ma; Early Magmatic Stage; Miller and Severson, 2002), small degree partial melts produced by deep melting of an enriched mantle plume source and variably mixed with melts from other sources (subcontinental lithospheric mantle?) were erupted as moderately differentiated plateau basalts (Fig. 6). The period from 1107 to 1102 Ma (Latent Magmatic Stage) is characterized by significantly diminished magmatism, consisting of mostly intermediate to felsic volcanics thought to record a period of regional underplating and crustal anatexis. Magma production reached a peak during regional extension of the Main Magmatic Stage (1102–1094 Ma) and is characterized by widespread, voluminous eruptions (up to 10 km thick) derived by shallow melting of asthenospheric mantle sources. Felsic volcanic and intrusive rocks that formed during this stage suggest greater contributions from the crust compared with those formed during early stage magmatism. The Late Magmatic Stage (1094–1086 Ma) is marked by decreased volcanic activity and increased deposition of thick fluvial and lacustrine sedimentary sequences (up to 8 km thick), perhaps recording plate drift away from the mantle plume source. Post-extensional compression, likely associated with Grenville tectonism, resulted in reactivation of earlier normal faults to form “tectonically-inverted” horsts along the southwest arm of the rift.

Figure 5.

Volcanic stratigraphy of the North Shore Volcanic Group. Modified from Davis and Green (1997) by Hoaglund (2010).

Figure 5.

Volcanic stratigraphy of the North Shore Volcanic Group. Modified from Davis and Green (1997) by Hoaglund (2010).

Figure 6.

Magmatic stages of Midcontinent Rift evolution. Modified from Vervoort et al. (2007) and Miller and Vervoort (1996).

Figure 6.

Magmatic stages of Midcontinent Rift evolution. Modified from Vervoort et al. (2007) and Miller and Vervoort (1996).

FIELD TRIP STOPS

Day 1

Drive north on I-35 to Exit 235 for Carlton, Minnesota. Turn east on Highway 210 and continue through the town of Carlton to a gravel parking area on the east side of the St. Louis River (∼3.5 mi east of I-35).

Stop 1-1. Slate and Greywacke of the Thomson Formation; Keweenawan Diabase Dikes

Location: Outcrops below dam at Thomson Reservoir Park at NAD83 UTM 15T 0545573E; 5168152N Overlook at NAD83 UTM 15T 0545579E; 5168016N

Goals

  • Become acquainted with Paleoproterozoic rocks of the Animikie Basin that underlie this portion of the Midcontinent Rift.

  • Characterize deformation associated with the foreland part of the Penokean fold-and-thrust belt.

  • Examine Keweenawan diabase dikes associated with the Midcontinent Rift.

Description

This is the type locality of the Thomson Formation, comprising a thick series of greywackes and slates that make up basement along the northwest limb of the Midcontinent Rift System. The Thomson Formation is regarded to be largely correlative with the Virginia Formation, exposed on the south side of the Mesabi Iron Range to the north, which in turn overlies the Biwabik Iron Formation and Pokegama Quartzite. Collectively, these rocks define the Animikie Basin and were deposited in a broad basin north of the leading edge of the north-vergent Penokean fold-and-thrust belt. Sediments deposited along the north margin of the basin (sandstone, shale, chert, and iron formation) were from the craton and largely deposited in shallow shelf environments. In contrast, the greywacke-slate sequences of the Thomson Formation were derived from immature arc environments and were deposited in submarine fan environments. Deformation and metamorphism of the basin sediments increases to the south. Detrital zircons from the Thomson Formation yield mostly 1800–2000 Ma U-Pb ages and a scattering of 2600-3600 Ma ages (Wirth et al., 2006). The youngest zircons observed in the Thomson Formation are ca. 1840 Ma, suggesting a maximum depositional age for the formation in this region.

Here, bedded greywacke, siltstone, and shale protoliths are weakly metamorphosed and deformed into gentle, east-west–trending open folds with near-vertical axial planar cleavage. North-vergent low-angle thrust faults can be mapped in this area. Bedding features (including graded beds, ripples, cross-beds, and load features) are well preserved and can be used to determine current and younging directions. Several near-vertical mafic (Fe-Ti tholeiitic basalt) dikes with well-developed chilled margins and subhorizontal cooling joints intrude the sedimentary layers of the Thomson Formation. The largest dike at this locality, and others nearby, have reversely polarized paleomagnetic poles and have been linked with volcanic flows of the Mesoproterozoic Midcontinent Rift on the basis of geochemical and paleomagnetic evidence (Green et al., 1987).

Opportunities for Student Learning. The diverse geological features and extensive exposures of bedrock at this stop afford a number of interesting pedagogical opportunities for students at all levels and in a range of geology courses. For students in introductory courses, the stop offers excellent examples of several important geological principles (sedimentary and igneous rocks, deformation) and the regional geology and tectonic evolution of northern Minnesota. For students in advanced courses such as field methods and structural geology, this locality can be used for extended mapping activities. The low-grade metamorphic rocks provide a good launching point for a metamorphic transect across the Penokean Orogen, which eventually leads to staurolite-zone rocks in the amphibolite facies near St. Cloud. The stop also makes an excellent locality for introducing advanced students to the Midcontinent Rift in petrology, geochemistry, and tectonics.

Example Learning Objectives. Students will be able to:

  • identify graded bedding and cleavage, and use them to define folds;

  • identify features (e.g., chill margins, cooling joints, contact metamorphism) and relate them to dike intrusion;

  • describe the petrology of the rocks of the Animikie Basin and explain their origin; and

  • make drawings showing the deformation associated with the Penokean fold-and-thrust belt and describe the forces that caused the deformation.

NEXT: Continue east (0.3 mi) on Highway 210, and then north on County Road 1 to I-35. Continue 3.8 mi north on I-35 to Midway Road (Exit 246). Drive south on Midway Road to Becks Road (County Highway 3) for 1.2 mi. Turn left (north) onto Skyline Drive for 2.3 mi to pull-off on right.

Stop 1-2. Basal Contact Zone to Lower Troctolite Zone of the Duluth Complex

Location: West Skyline Parkway, south of Bardon Peak Park at NAD83 UTM 15T 0558543E; 5170342N Begin at NAD83 UTM 15T 0558497E; 5170240N End at NAD83 UTM 15T 0558772E; 5170475N

Goals

  • Describe and classify rocks of the lower Duluth Complex.

  • Characterize the nature of igneous layering.

  • Seek evidence bearing on the origins of igneous layers in the Duluth Complex.

Description

The Duluth Complex is one of the world’s largest layered mafic intrusions. It is dominated by cumulate gabbro and troctolite, and large masses of anorthosite. This stop offers an opportunity to examine layered igneous rocks near the base of the Duluth Layered Series (Miller et al., 1995). Roadside outcrops and nearby knolls expose layered gabbro, troctolite, and peridotite bodies. From a rocky knoll south of the vehicle pull-off, the moderately dipping (45° east) basal contact of the Duluth Complex can be observed ∼500 m to the west. Beneath the Duluth Complex are shallow-dipping (15° east) flows of reversely polarized Ely’s Peak basalt (formed during the Early Magmatic Stage of rift evolution between 1109 and 1107 Ma). Near the contact with the Duluth Complex, the basalt is recrystallized to a fine-grained hornfels. The slopes down to the railroad tracks below the overlook expose small crosscutting bodies of biotitic oxide dunite to peridotite and are typical of the 200–300 m thick Lower Contact Zone (Miller and Green, 2008).

The Troctolite Zone (1000–1500 m thick) of the Duluth Layered Series comprises augite ophitic troctolite, troctolite, and melatroctolite that display layering of several types (modal, textural) and scales (cm to meter). Mineral analyses reveal limited cryptic compositional variations. Some layers pinch out along strike and may record trough layering. Combined, these features are interpreted to record the early crystallization history of the Duluth Layered Series magma chamber, including processes of crystallization, turbulent convection, magma reinjection, and volatile fluxing from the footwall (Miller, 1995b).

Walk east along Skyline parkway to the overlook at a sharp bend in the road (∼300 m). Rock exposed on the inside of the bend is augite troctolite with isomodal olivine layering. To the east and south is a view of the drowned estuary of the St. Louis River. During Late Wisconsin glaciation, water levels in glacial Lake Duluth rose to more than 165 m above current lake levels when eastward drainages were dammed by retreating ice. During this time (∼12,000 years ago), glacial meltwater drained southward through the Brule and St. Croix rivers. After retreat of the ice to the north, an eastward drainage was established through the Straits of Mackinac. Uplift in the northeast, due to glacial rebound, is causing Lake Superior to tilt to the south, resulting in drowning of the mouth of the St. Louis River. Water levels in this part of the lake are estimated to be rising at a rate of 15 cm per century (Ojakangas and Matsch, 1982).

Opportunities for Student Learning. The excellent exposures of the layered mafic rocks in this part of the Duluth Complex provide a great setting for mineralogy and/or petrology classes. For introductory students, this stop provides an opportunity to examine coarse-grained igneous rocks in the field, and to relate rock composition to igneous processes (fractional crystallization, crystal settling). Students in petrology courses can practice skills in rock identification and descriptions, and then use these observations through the short transect to determine petrologic variations, which in turn leads to crystallization processes (crystal settling, fractionation, magma recharge, etc.). Adjacent outcrops provide a good setting for small group assignments, coupled with discussion between groups about the findings. Recognition of layering can also lead to exploration of additional lab tools that could be used to address petrogenetic and fractionation process (petrography, mineral compositions, geochemistry, isotope geochemistry, etc.).

Example Learning Objectives. Students will be able to:

  • describe small and large-scale features of layered igneous rocks;

  • sketch a cross-section illustrating the relationships among rock types near the base of the Duluth Complex; and

  • relate mineral textures and rock compositions to Bowen’s reaction series and mechanisms of fractionation.

NEXT: Continue east on West Skyline Parkway to I-35. Turn right (north) on I-35 and follow MN-61N through Duluth. Just exiting the city, turn right onto Scenic Highway 61 (Congdon Boulevard) and drive northeast ∼1.2 mi to the third wayside rest on the east side of the highway. Park vehicle and walk southwest along road to large grassy opening on SE side of highway. Follow small unmarked trail to shoreline.

Stop 1-3. Volcanic Flows of the Lakewood Lava Sequence

Location: Scenic Highway 61 NE of Kitchi Gammi Park Park near NAD83 UTM 15T 0577903E; 5189378N Begin traverse at NAD83 UTM 15T 0577689E; 5189172N

Goals

  • Examine and describe diverse rock types along shoreline traverse.

  • Hypothesize their origins and relationships.

  • Observe spectacularly preserved flow features along traverse.

Description

Much of the picturesque rocky shoreline on the Minnesota side of Lake Superior is due to the innumerable basalt flows of the North Shore Volcanic Group, which form prominent ledges sloping gently toward the water. The structures of several flows can be examined at this nearly continuous lakeshore exposure. The flows in this part of the rift are part of the upper southwest sequence of flows exposed along the North Shore of Lake Superior. The flows of this specific locality are part of the normally polarized Lakewood lavas, a 1280-m-thick sequence of mostly basalt with ferroandesite, icelandite, and rhyolite near the base. Geochronologic studies by Davis and Green (1997) constrain the ages of these to 1098–1096 Ma. This unit, and the thick lava sequences exposed farther up section, was erupted during the main stage (1100–1094 Ma) of rift evolution (Miller and Vervoort, 1996).

After walking southwest of the parking area and descending though pink-tinted clays of the Glacial Lake Duluth to the lakeshore, the stop begins in rhyolite with highly contorted flow banding. The 80-m-thick (13-km-long) rhyolite is overlain by discontinuous, thin interflow sandstone. The contact between the overlying basalt with the rhyolite and sandstone is highly irregular and complex, and amygdule fillings of abundant calcite and rare purple fluorite are present. Walking up section (northeast) along the shoreline permits examination of four basalt flows that exhibit well-developed flow features, including pipe vesicles, vesicle cylinders, amygdaloidal zones (upper flow), and ropy pahoehoe on the upper surfaces. Alteration minerals in these flows are characteristic of a chlorite-zone assemblage containing laumontite-albite-chlorite ± prehnite ± pumpellyite that reflects temperatures of 220–260 °C associated with burial metamorphism in this portion of the rift (Schmidt, 1990, 1993; Schmidt and Robinson, 1997).

The Lakewood basalt flows are weakly alkaline to olivine tholeiites and are characterized by diabasic textures; primitive tholeiites are more abundant near the top of the sequence. Nd isotopic studies indicate that felsic lavas in this sequence have different origins. Rhyolites have more radiogenic compositions, suggesting the involvement of Archean crust, whereas icelandites have only negative εNd compositions thought to reflect subcontinental lithospheric mantle (Vervoort and Green, 1997). Mafic flows erupted during the main stage of rifting are characterized by primitive mantle compositions (εNd ∼0) thought to reflect a shift to large degree partial melting of a mantle plume source following a latent stage of crustal heating (Shirey et al., 1994; Miller and Vervoort, 1996; Nicholson et al., 1997).

Opportunities for Student Learning. This is a good place for students to confront ambiguity! The geologic relations, although relatively well exposed, offer a good opportunity to observe, discuss, and interpret the various contact relations. Sources of ambiguity include: (a) units of different composition have similar red color; (b) distinguishing fine-scale laminations as either sedimentary bedding or volcanic layering; and (c) interpreting contact relations as intrusive or by flow. In short, outcrop relations such as this help students determine the number of units present and their age relations, as well as developing ideas about process of formation. Farther along to the NE, there are good examples of less ambiguous features (pahoehoe flow tops) that can help to build confidence solidify understanding of flow features.

Example Learning Objectives. Students will be able to:

  • describe the differences in character of mafic and felsic flows;

  • make a sketch of a generalized mafic flow illustrating the distribution of features;

  • characterize the alteration minerals present, their distributions within flows, and the overall metamorphic grade; and

  • develop a strategy for sampling for characterizing the major and trace element whole-rock compositions of this sequence of flows.

Return upslope to parking area along south side of small creek.

NEXT: Drive northeast (∼0.3 mi) along Scenic Highway 61 to wayside rest opposite Lake Shore Cottages Resort. Park and descend to lakeshore.

Stop 1-4. Interflow Sandstone

Location: Wayside rest along Scenic Highway 61; opposite North Shore Cottages Park at NAD83 UTM 15T 0578316E; 5189656N

Goals

  • Examine interflow sediment.

  • Explore significance of sedimentary features for volcanic setting and processes.

Description

In between basalt flows of the North Shore Volcanic Group are occasional thin units of ferruginous, eolian siltstone and fine-grained sandstone. Typically these units form discontinuous sheets, and locally they show large cross-bedded sedimentary structures indicating an eolian, terrestrial depositional environment. They are compositionally immature and consist predominantly of plagioclase, rock fragments (mafic volcanics), and minor quartz and opaque minerals (Jirsa, 1984). Planar and trough cross beds indicate an easterly paleocurrent direction. This interflow sediment is underlain by an icelandite and is overlain by a basaltic andesite. With rare exception, detrital zircons in this interflow sediment have ages of ca. 1100 Ma, suggesting that the interflow sediments were derived largely from juvenile material from within the rift (Wirth et al., 2006); rare Penokean and Archean grains make up the remainder of the zircon population.

Opportunities for Student Learning. Many field trips along the North Shore will focus on igneous petrology, but this stop provides a digression into the sedimentary record of basin fill within the Midcontinent Rift. Although interflow sediments are intermittent in this part of the rift succession, they dominate the upper one-third of the total rift infill. This stop serves as an example of a brief stop that can be made to help students complete the tectonic picture of rift evolution. A number of lines of questioning could be explored here. What is the depositional setting? What was the paleogeographic setting of surficial environment of the Midcontinent Rift? What would it have been like to stand here in the rift one billion years ago? What is the composition of the sediment (immature)? Why? How could we use the sediments here to help elucidate the paleogeography and/or surface process?

Example Learning Objectives. Students will be able to:

  • describe the mineralogical and textural features of an interflow sediment; and

  • formulate hypotheses about the nature of the rift from the sediments.

NEXT: Drive northeast along Scenic Highway 61 to South Lakewood Road, turn left (northwest) and proceed to intersection with MN-61 (expressway). Turn right (northeast) and follow MN-61N for ∼42 mi through Two Harbors to Beaver Bay. Park in pull-off on right side of highway after crossing bridge over the Beaver River. Follow trails to ENE to shoreline on north side of rock promontory.

Stop 1-5. Beaver River Diabase with Varied Inclusions and Intrusive Relations

Location: Shoreline north of the Beaver River

Park at NAD83 UTM 15T 0629024E; 5235589N

Begin at NAD83 UTM 15T 0629111E; 5235578N

End at NAD83 UTM 15T 0629231E; 5235806N

Goals

  • Examine various rock types and geologic relations;

  • Generate hypotheses of their origins and relations; and

  • Propose tests that could be used to support hypotheses.

Description

Acting as a widespread magmatic screen between the lower Duluth Complex and the upper North Shore Volcanic Group, the Beaver Bay Complex represents one of the youngest intrusive groups within the Midcontinent Rift. The sequence of shoreline outcrops at this locality illustrates the complex mixture of rock types and intrusive relations that are typical of the lower Beaver River diabase, one of the most extensive units in the Beaver Bay Complex. The traverse begins on the north side of the rocky cliff at the north end of the sandbar at the mouth of the Beaver River. Here, a large xenolith of quartz-feldspar porphyritic rhyolite is exposed. This rhyolite is similar to the rhyolites exposed at Palisade Head and Shovel Point farther to the north.

Farther northeast along the shoreline is a series of beach-level outcrops that expose a variety of deep-green altered anorthosite inclusions in altered diabase. Farther northward, these rocks give way to fine-grained diabase that hosts numerous straight to curvilinear granophyre dikes of varying size. Other rocks in this area appear to be basaltic hornfels, notably with dark-red rims. Continuing northeast over a prominent rocky point, the diabasegranophyre-basalt assemblage gives way to diabase with abundant and large, light-colored anorthosite inclusions. This association of rock types is characteristic of the Beaver River diabase and can also be observed at the historic Split Rock Lighthouse State Park.

Opportunities for Student Learning. The rock types exposed at this stop are varied, and the relations among them complex, challenging even the most advanced students. This stop provides an opportunity to describe and interpret a number of different rock types over a short distance, and offers opportunities to map their distribution and relations, hypothesize their origins, and design tests of those hypotheses. As with many good examples elsewhere of igneous cross-cutting relations, students can be asked simply to define the number of geologic units and their age sequence; this could be combined with detailed mapping, graphical illustration of the relationships, erection of a geologic history, etc.

Example Learning Objectives. Students will be able to:

  • describe phenocrysts in rhyolite and explain their significance;

  • explain the contact relations that characterize xenoliths and dikes; and

  • identify polysynthetic twinning in plagioclase.

From here, head up-slope a short distance through the woods to the highway, and walk southwest back to vehicle.

NEXT: Continue northeast ∼3.3 mi along MN-61N to Bus Park road on the right. Follow Bus Park to the first dirt road on the right (∼0.1 mi) and then follow an unnamed dirt road to East Lakeview Drive. Turn left (northeast) and follow East Lakeview Drive ∼0.2 mi to old Silver Bay Gun Club (abandoned white building). Walk east to a small cove behind the white building.

Stop 1-6. Silver Beaver Rhyolite

Location: Lakeshore south of Williams Creek

Park at NAD83 UTM 15T 0632939E; 5240277N

Outcrops near NAD83 UTM 15T 0633006E; 5240208N

Goals

  • Examine features of a felsic flow.

  • Consider petrographic and phase relations.

Description

This stop provides an opportunity to examine a well-exposed rhyolite with well-developed internal flow laminae. This rhyolite, situated within the Baptism River lava sequence (Fig. 5), named the Silver Beaver rhyolite by Green and Fitz (1993), is ∼100 m thick and can be traced laterally for ∼15 km along strike. Although aphyric in hand specimen, thin-sections of these rocks contain a felty, finegrained groundmass of poikilitic quartz and alkali feldspar (Green and Fitz, 1993). The quartz grains consist of paramorphs after tridymite. The bottom of this unit contains a brecciated vitric zone and the top of the unit exhibits a flow-top breccia with lineated amygdules and angular chips of devitrified glass. Pyroclastic facies features are absent and the unit is interpreted to be a near-liquidus temperature flow (Green and Fitz, 1993).

A second, well-exposed rhyolite, the Palisade rhyolite (Fig. 5), overlies the Silver Beaver rhyolite and is prominently exposed along the North Shore just to the north of this stop. The Palisade rhyolite is similar to the Silver Beaver rhyolite in thickness and lateral extent, but has a welded tuff along its base and an auto-clastic breccia (consisting of folded, laminated blocks of rhyolite) at its top. These features suggest that the Palisade rhyolite originated as a rheoignimbrite that welded and began to flow (Green and Fitz, 1993).

Opportunities for Student Learning. Discussions of both physical volcanology and melt sources are appropriate at a stop such as this. This stop, and at other rhyolite exposures along the North Shore, provides an opportunity for introductory and advanced students to examine felsic flow features and consider the petrographic evidence for high-temperature felsic magmatism along the northwest limb of the Midcontinent Rift. Features such as folded flow banding provide a good opportunity to discuss volcanic-flow versus tectonic origins of folds. The chemical composition of bimodal rift volcanism also provides an opportunity to explore the sources of melting. Most of the rift is composed of tholeiitic basalt, but a small fraction of Fe-rich rhyolite (including icelandites) indicates some degree of melting of older, pre-rift continental crust. Radiogenic isotope compositions bear out this interpretation and can be used to extend the field geologic relationships.

Example Learning Objectives. Students will be able to:

  • describe features related to felsic flows; and

  • explain the mineral compositions of North Shore Volcanic Group rhyolites using phase diagrams.

NEXT: Drive northeast on East Lakeview Drive. Turn right (northeast) on MN-61N and drive ∼5.5 mi to a gentle bend in the road that curves east. Park opposite low outcrops on west side of road.

Stop 1-7. Basalt Flows with Complex Volcanic and Structural Features

Location: South of Kennedy Creek along Highway 61 Park at NAD83 UTM 15T 0638150E; 5247660N

Goals

  • Make outcrop sketch.

  • Recognize flow characteristics.

  • Determine number of flows and stratigraphic “up.”

Description

A low roadcut exposes a series of basalt lava flows of the Bell Harbor sequence (Fig. 5), here dipping more steeply than elsewhere in the North Shore Volcanic Group. The basalts range from olivine tholeiites, with smooth upper flow surfaces and pipe amygdules at their base, to quartz tholeiites with brecciated (aa) flow tops. They show typical features of the North Shore flow units. Flows here range from 3 to 20 m in thickness and they include thin interflow sediment units (mainly argillite to fine sandstone).

The geologic relations in this area are highly complex compared to other parts of the North Shore region, mainly because the volcanic units were highly disrupted during emplacement of the Beaver River diabase and other intrusions of the Beaver Bay Complex. Intrusion of the diabase caused block faulting, displacement, and rotation of the volcanic rocks.

Opportunities for Student Learning. This is an excellent stop to test student (introductory and advanced) observational skills and build on what has been learned at previous outcrops. The main point of a stop like this is to introduce a higher level of complexity (i.e., that these flows are overturned). A workable approach, assuming that students have already been exposed to the “typical” features of basalt flows in this region (e.g., Stop 1-3), is to have them make a careful profile sketch and interpretation of the roadcut. An assignment might be to make a sketch that resembles a graphic log of the flow units, including details of the various flow features encountered. Careful observation of subtle variations in flow features will indicate that the flows are upside-down, which makes a good starting point for group discussion about the origin of this unusual occurrence.

Example Learning Objectives. Students will be able to:

  • make a sketch map to illustrate the geology of an outcrop; and

  • identify flow features and identify flow tops and bottoms.

NEXT: Drive ∼0.3 mi northeast along MN-61N to the north end of large roadcuts on both sides of road.

Stop 1-8. Diabase with Large Anorthosite Inclusion

Location: South of Kennedy Creek along Highway 61 Park at NAD83 UTM 15T 0638455E; 5247906N ‘

Goals

  • Examine ophitic diabase.

  • Generate hypotheses (and tests) for origin(s) of large anorthosite xenolith.

Description

This is one of several spectacular exposures of anorthosite along the North Shore of Lake Superior. Other exposures not visited on this trip include Carlton Peak, Split Rock Lighthouse State Park, Silver Bay, and Tettegouche State Park.

The anorthosite xenolith at this stop has an outcrop length of nearly 40 paces and is hosted by diabase (Boerboom et al., 2004). South of the anorthosite, the diabase is medium to coarse grained with augite oikocrysts up to 5 cm in diameter. North of the anorthosite, the diabase hosts inclusions of hornfels basalt and is intruded by ferromonzodiorite and granophyric dikes.

The anorthosite consists almost entirely of coarsegrained plagioclase (bytownite to labradorite up to 20 cm long) with rare olivine, orthopyroxene, and clinopyrox-ene. As with other anorthosite inclusions along the North Shore, the surrounding diabase is not chilled against the inclusion. Furthermore, the anorthosite itself is homogeneous and unlayered. Together with the highly disordered state of the plagioclase in the anorthosite inclusions, these features suggest that the anorthosite inclusions may have originated by widespread, high-pressure fractionation and floatation of plagioclase in mid- to lower-crustal magma chambers during the early phase of rifting (Miller and Weiblen, 1990).

Opportunities for Student Learning. Some geologic stops are just for fun, and this stop never fails to remind all of us why we love geology. Students, in both introductory and advanced courses, not only respond to the aesthetic beauty of the rock, but they also cannot but wonder about the origins of the large crystals in this unusual monomineralic rock. An instructor can also use the occasion to both inculcate enthusiasm for geology, discovery and the wonders of nature. This is also a great place to collect a sample for display, for making bookends, or for a gift. Even if students were never quite confident that they could identify plagioclase in the field, they delight in being able to put their knowledge and skills into practice. The petrogenesis of anorthosites can also be discussed in more detail, even though this is an isolated block within diabase.

Example Learning Objectives. Students will be able to:

  • identify polysynthetic twinning in plagioclase; and

  • explain why anorthosites are uncommon and require special conditions of formation.

NEXT: Drive ∼12.0 mi northeast on MN-61N to a parking area for Sugarloaf Cove on the right (just past Sugarloaf Road on left).

Stop 1-9 (Optional Stop). Olivine Tholeiite Basalt Flows

Location: Sugarloaf Cove

See also: Sugarloaf: The North Shore Stewardship Association (http://www.sugarloafnorthshore.org/index.php)

Park at NAD83 UTM 15T 0651753E; 5261391N

Outcrops near NAD83 UTM 15T 0652010E; 5261130N

Goals

  • Observe tholeiitic lava flows and pahoehoe structures.

  • Find evidence of burial metamorphism (zeolite-facies amygdules).

  • Determine paleoenvironment of volcanism (pipe vesicles, clastic dikes, pahoehoe flow tops, polygonal joints).

  • Determine paleoflow direction.

Description

Sugarloaf Cove is a small natural harbor used in the late nineteenth and early twentieth centuries as a logging outpost. Horse-drawn timber was harvested in winter and brought to the cove, where it was assembled after ice breakout into large boomed rafts for transport to sawmills across Lake Superior. Remnants of the logging camp and hardware used to skid timber are still visible today.

A sequence of gently dipping olivine tholeiite basalt flows (Fig. 7) of the North Shore Volcanic Group are exposed along the inner cove shoreline. These flows represent the uppermost eruptive sequence of the group, referred to as the Schroeder-Lutsen sequence (Fig. 5). Flows here range from several tens of meters thick (with ophitic texture displayed in massive flow interiors) to thin flow units of a meter or less. Here you will find large zeolite-filled amygdules on sloping flows of the inner bay. Cobbles and boulders on the beach (a tombolo) connecting the mainland to the peninsula of Sugarloaf Point are highly diverse rock types (light-colored Archean granites and gneisses from the Canadian part of the Superior province; black basalts of local origin; red basalts and granophyres from inland; trachytic intrusive rocks; etc.), reflecting different sources of glacially derived material that is reworked during winter storms. Outcrops on the peninsula consist of a thin flow sequence showing ropy pahoehoe flow tops, pipe vesicles, and clastic dikes.

The thin olivine tholeiite flows that form the inner harbor slope are adorned with numerous large (≤4 cm) amygdules in the thomsonite-scolecite-smectite zone (Schmidt and Green, 1992; Schmidt, 1993). Other amygdule minerals include calcite, heulandite, laumontite, and stilbite. These mineral alteration assemblages indicate the basalt flows here reached the lower zeolite facies of burial metamorphism. Farther to the southwest toward Duluth, the flows are structurally deeper and reach prehnite-pumpellyite and lower greenschist facies metamorphism. Concomitant with upward changes in mineral assemblage are gradational changes in mineral composition, degree of albitization, and O- and C-isotopic values of calcite. The general trends across the volcanic pile are often repeated within single flow units, interpreted as a function of rock permeability.

Because this is a protected State Scientific and Natural Area, please abide by the “no hammers” policy!

Opportunities for Student Learning. This is a marvelous place for a walking stop, which can take up to a couple of hours. Introductory students find it a good mix of history, culture, natural science, igneous geology, mineralogy, volcanology, geomorphology, and beach combing. For a petrology class, the inner cove outcrops can be used to set up discussion of burial metamorphism, alteration, and fluid migration in a volcanic pile. Out on the peninsula, a series of thin basalt flows provide relationships related to consideration of eruptive environment. Are these flows subaerial or submarine? Are there paleoflow indicators that can be used to reconstruct paleoenvironment? Are they consistent? A variety of activities are possible at a stop like this–including detailed mapping of flows, measuring paleoflow indicators (pipe vesicles, ropy pahoehoe), or just by way of compiling geologic evidence for a geologic history and/or eruptive setting.

Example Learning Objectives. Students will be able to:

  • use flow features to identify flow tops and bottoms;

  • characterize the conditions of metamorphism; and

  • reconstruct the volcanic environment during rifting.

NEXT: Return southwest along MN-61N to AmericInn Lodge and Suites in Silver Bay (∼18.2 mi).

Figure 7.

Idealized internal structure of olivine tholeiite-composition lava flows. From Green (1989).

Figure 7.

Idealized internal structure of olivine tholeiite-composition lava flows. From Green (1989).

Day 2

The focus of Day 2 stops will be a detailed look at a spectacularly layered mafic intrusion in the Beaver Bay Complex, the Sonju Lake Intrusion, and the associated Finland Granophyre (Fig. 4). From the AmericInn in Silver Bay, drive northeast on MN-61N. Turn left (northwest) on MN-1W toward Finland, Minnesota. After crossing the Baptism River in Finland, turn right (northeast) on Cramer Road (County Road 7). Turn left on Sonju Lake Forest Road after 6.7 mi, and continue on Sonju Lake Road to an unmarked dirt road on right (∼3.1 mi after leaving Cramer Road).

Stop 2-1. Lower Cumulates of the Sonju Lake Intrusion

Location: Traverse (2.0 mi round-trip) to base of Sonju Lake

Intrusion; west of Lake Twentythree

Park at NAD83 UTM 15T 0636160E; 5260616N

Northern end of traverse at NAD83 UTM 15T 0635942E; 5262020N

Goals

  • Examine basal units of the Sonju Lake Intrusion.

  • Use field and petrologic evidence to hypothesize models for their origin.

Description

This two-mile round-trip traverse crosses the lower units and basal contact zone of the Sonju Lake Intrusion, a layered intrusive of the Beaver Bay Complex. The route begins along an unnamed dirt road that leads north (over Beaver River diabase along the eastern margin of the intrusion) of the Sonju Lake Road, and then follows an unmarked route through the woods. The group will walk to the northernmost end of the traverse and then examine geology (working up section) along a return route to the west of the logging road. There are many scattered outcrops throughout the woods. Key localities are indicated on the map (Fig. 8) and described below.

Region A (near UTM 15T 0635942E; 5262020N)

The stop begins in coarse-grained olivine gabbro of uncertain affinity that comprises the country rock in this portion of the intrusion. In the laboratory, these rocks look mineralogically and texturally similar to some gabbroic rocks higher up in the Sonju Lake Intrusion. Starting at this locality gives students a chance to observe the textural changes that occur as one traverses into the lower Sonju Lake Intrusion.

Figure 8.

Traverse map for lower Sonju Lake Intrusion. Modified from Miller et al. (1993a). Unit symbols (oldest to youngest): og—early olivine gabbro or Beaver Bay Complex; bld—Blessner Lake diabase; sldp—SLI diabasic picrite; sld—SLI dunite; slt—SLI troctolite; slg—SLI gabbro; slfg—SLI ferrogabbro; dgh—diorite-granophyre hybrid; brd—Beaver River diabase.

Figure 8.

Traverse map for lower Sonju Lake Intrusion. Modified from Miller et al. (1993a). Unit symbols (oldest to youngest): og—early olivine gabbro or Beaver Bay Complex; bld—Blessner Lake diabase; sldp—SLI diabasic picrite; sld—SLI dunite; slt—SLI troctolite; slg—SLI gabbro; slfg—SLI ferrogabbro; dgh—diorite-granophyre hybrid; brd—Beaver River diabase.

Region B (near UTM 15T 0635868E; 5261971N)

This locality is located within the basal diabasic picrite (sldp unit) of the Sonju Lake Intrusion. The contact between the melatroctolitic rocks of the basal Sonju Lake Intrusion and the underlying gabbros is not well exposed here, but a sharp contact is well exposed in a nearby drill-core that penetrates the footwall. Modal layering and lamination is generally poorly developed, especially in the lower part of the unit. The abundant olivine in these rocks makes them susceptible to pervasive serpentinization. The noticeably finegrained texture of the rocks at this stop contrasts with the overlying rocks of the Sonju Lake Intrusion and provides an opportunity to discuss textures, thermal histories, and intrusion geometry.

Region C (near UTM 15T 0635749E; 5261813N)

Traversing higher into the intrusion, the predominant rock type becomes medium- to coarse-grained dunite (sld unit). Together, the lower diabasic picrite (sldp) and dunite (sld) units are somewhat problematic in the field and in the laboratory. The abundance of plagioclase in the lowermost unit, and general lack of plagioclase in the overlying dunite, does not readily fit with predictions from phase diagrams. Furthermore, students realize that phase diagrams assuming equilibrium processes do not predict the complex and cyclical compositional changes observed in the dunite unit.

Region D (near UTM 15T 0635619E; 5261677N)

This stop is within the lower part of the troctolite unit (slt), and the rocks at this level of the Sonju Lake Intrusion are noticeably more laminated, coarse-grained, and plagioclase rich. Rhythmic layering is especially well pronounced in the lower part of the section, and curved structures reminiscent of cross-bedding are locally present. Cumulate olivine and plagioclase are joined by ophitic clinopyroxene that becomes more abundant (and larger) up section. Olivine-rich rocks are typically more intensely serpentinized. These rocks provide an ideal opportunity to apply theoretical phase diagrams to understanding the textures and progression of rock types found in the lower part of the Sonju Lake Intrusion.

Opportunities for Student Learning. In contrast to the typical roadside geology stops, this is a great example of how students can learn at a “geologic pace” associated with walking across a well-defined traverse. An instructor has to balance keeping the group together and moving at a reasonable pace, with allowing for individual discovery and personal contact with all students. Instructors will also have to balance the urge to “get to the next stop” with allowing for a more deliberate pace of observation, questioning, reflection, and being on one’s own in the woods. Learning objectives previously used at this stop include: (a) using it as a descriptive and sampling traverse, as would be done during a research or exploration study; (b) making comparisons of variable rock compositions over a distance and recognize subtle variations; (c) comparing field geology with thin sections and geochemical data; (d) defining the field geologic relations necessary as a basis for first-order interpretation of petrologic process; and (e) working as a group. Specific learning goals are outlined in the different subareas of the traverse, as noted above.

Example Learning Objectives. Students will be able to:

  • identify common minerals and rock types in the lower part of the Sonju Lake Intrusion;

  • locate themselves on a topographic map;

  • measure the orientation of igneous layering and mineral foliation;

  • explain the origins of layered igneous rocks using Bowen’s Reaction Series; and

  • estimate the composition of the initial SLI magma prior to differentiation.

Although it is possible to continue up section (south) from this point, it is difficult to cross the East Branch of the Baptism River in this area. Instead, we will return east through woods to the unnamed dirt road. Turn south and follow the road to the Sonju Lake Road and vehicle.

NEXT: Continue west along the Sonju Lake Road ∼0.2 mi to next stop.

Stop 2-2. Middle cumulates of the Sonju Lake Intrusion

Location: Traverse (0.6 mi round-trip) south of Sonju Lake Road; east of Sonju Lake

Park near NAD83 UTM 15T 0635792E; 5260684N

Traverse to NAD83 UTM 15T 0635672E; 5260139N

Goals

  • Examine middle units of the Sonju Lake Intrusion.

  • Use field and petrologic evidence to hypothesize models for their origin.

Description

This traverse takes us through the middle cumulate layers of the Sonju Lake Intrusion (Fig. 8). Rocks near the parking area along the Sonju Lake Road are in the uppermost part of the troctolite unit (slt unit of Miller et al., 1993b). The traverse then crosses over successively higher units consisting of gabbro (slg) and ferrogabbro (sfg; Miller et al., 1993b). Near the beginning of the traverse (slt unit), cumulus olivine and plagioclase occur with intercumulus augite and Fe-Ti oxides. Farther south, the appearance of cumulus pyroxene marks the boundary of gabbro (slg). The boundary of the ferrogabbro unit (slfg) is marked by the appearance (and increased modal abundance) of cumulus Fe-Ti oxides and the nearly ubiquitous disappearance of olivine. Mineral and whole rock compositions exhibit unidirectional and gradual compositional changes throughout this portion of the intrusion. Farther up section (southeast) the rocks grade successively to apatite-bearing olivine diorite (slad unit) and olivine ferromonzodiorite (slmd unit) and can be seen along the optional Stop 2-3.

Opportunities for Student Learning. This traverse provides students an opportunity to apply their petrographic skills and knowledge of phase equilibria to understanding mineral abundance and rock textures in the field, and to draw comparisons with other well-studied layered mafic intrusions (e.g., Skaergaard, Bushveld).

Example Learning Objectives. Students will be able to:

  • compare and illustrate rock textures exhibited by the troctolite and gabbro units of the Sonju Lake Intrusion;

  • explain rock compositions and textures in the lower Sonju Lake Intrusion using the Fo-An-Di phase diagram; and

  • describe the whole-rock composition of this rock and its significance to the evolving Sonju Lake Intrusion magma system.

NEXT: Retrace route back toward Finland following Sonju Lake Road and County Highway 7 (Cramer Lake Road). Just after the pavement begins, turn right (northwest) and follow Airbase Road 1.4 mi toward the site of the former U.S. Air Force Radar Station. Pull off to the left near the crossing with a gravel road.

Stop 2-3 (Optional Stop). Upper Half of Sonju Lake Intrusion and Lower Finland Granophyre

Location: Traverse (2.5 mi) along an abandoned logging road west of Sonju Lake

Park at NAD83 UTM 15T 0633965E; 5260602N

Traverse to NAD83 UTM 15T 0632831E; 5257264N near decommissioned Finland radar towers

Goals

  • Examine upper units of the Sonju Lake Intrusion and Finland Granophyre.

  • Use field and petrologic evidence to hypothesize models for their origin.

Description

This traverse along an abandoned logging road (Lookout Road) crosses units of the upper half of the Sonju Lake Intrusion and the lower portion of the Finland Granophyre (Fig. 9). Below is a summary of the detailed description by Miller (1995c). This traverse can be challenging for students because of the highly variable rock compositions and textures in the upper part of the section, and because of the gradational nature of the contacts.

The traverse begins in the lower part of the gabbro unit (slg; Miller et al., 1993b), just above the contact with the troctolite unit (slt). The boundary between these two units is marked by the transition from cumulus plag + ol to plag + cpx + ol. Very quickly, the road crosses the boundary between the gabbro (slg) and ferrogabbro (slfg) units, marked by the appearance of cumulus Fe-Ti oxides (pl + cpx + ox ± ol); olivine is generally absent and inverted pigeonite is locally present in the lower part of the section. The ferrogabbros may contain up to 25% subhedral ilmenite-titanomagnetite and are well laminated, but typically lack pronounced compositional layering (area “B”; near NAD83 UTM 15T 0633801E; 5260466N)

After traversing the middle portion of the ferrogabbro unit, the road crosses a hybrid diabasic dike (near area « H »; NAD83 UTM 15T 0634403E; 5259382N) thought to be unrelated to the Sonju Lake Intrusion (Miller, 1995c). Soon after crossing the dike, the traverse passes over apatite-bearing olivine ferrodiorite (slad unit), marked by the appearance of cumulus apatite and reappearance of olivine (pl + cpx + ox + ol + ap) with interstitial amphibole and granophyric intergrowths. An excellent exposure of this unit can be found at area “I” on the traverse map (near NAD83 UTM 15T 0634271E; 5259145N). The sample for U-Pb dating of the Sonju Lake Intrusion (1096 ± 0.8 Ma; Paces and Miller, 1993) comes from near this location.

Next, the road crosses over the olivine ferromonzodiorite unit (slmd). Rocks of this unit are characterized by varied textures and compositions that range from apatite-bearing olivine ferrodiorite to olivine ferromonzodiorite (e.g., near area “J”; NAD83 UTM 15T 0634180E; 5258970N). The amount of granophyre mesostasis is highly variable. Evidence from a nearby drill core through this portion of the section suggests that the slad and slmd end-members are cyclically interlayered, and that complex hybrid rocks may have been produced by interaction with the overlying Finland Granophyre (Miller, 1995c).

The boundary with the Finland Granophyre is gradational and irregular, and is mapped on the basis <25% mafic minerals. The basal unit of the Finland Granophyre is quartz ferromonzodiorite (frpm) consisting of plagioclase with myrmekitic overgrowths, prismatic iron-rich clinopyroxene, quartz, and a micrographic mesostasis consisting of quartz and alkali feldspar (area “O”; near NAD83 UTM 15T 0634028E; 5258523N). Next, the road passes over low exposures of leucogranite (e.g., near NAD83 UTM 15T 0633349E; 5257618N), the uppermost, and main phase (frg) of the Finland Granophyre. The rock is typically massive, consisting of altered feldspars and <5% mafic minerals in a matrix of micrographic quartz and feldspar inter-growths. This unit is better exposed in the extensive road-cuts at the next stop (2-4).

Opportunities for Student Learning. The field and geochemical characteristics the Finland Granophyre and Sonju Lake Intrusion are complicated and present an ideal opportunity for students to explore evidence in support of the possible models for the origin(s) of these two intrusive bodies. Evidence from exposures across the contact zone is ambiguous. What other petrologic, geochemical, or geochronological evidence could be used to illuminate their origins and relationship?

Example Learning Objectives. Students will be able to:

  • use an outcrop map to locate themselves in the field;

  • describe the changes in rock compositions and textures across the contact zone between the Sonju Lake Intrusion and the Finland Granophyre; and

  • evaluate various possible models for the origins of the Sonju Lake Intrusion and Finland Granophyre using field and petrologic evidence.

Figure 9.

Traverse map through upper part of Sonju Lake Intrusion and lower Finland Granophyre (Miller, 1995c). Route follows unnamed logging road. Outcrops along road are indicated with solid black fill. Units (oldest to youngest): frpm—quartz ferromonzodiorite (Finland Granophyre); frg—leucogranite (Finland Granophyre); slt—SLI troctolite; slg—SLI gabbro; slfg—SLI ferrogabbro; dgh—diorite-granophyre hybrid; brd—Beaver River diabase. Locations of drill cores (DDH1, DDH2, DDH3) through sulfide horizon (narrow-spaced vertical lines)

Figure 9.

Traverse map through upper part of Sonju Lake Intrusion and lower Finland Granophyre (Miller, 1995c). Route follows unnamed logging road. Outcrops along road are indicated with solid black fill. Units (oldest to youngest): frpm—quartz ferromonzodiorite (Finland Granophyre); frg—leucogranite (Finland Granophyre); slt—SLI troctolite; slg—SLI gabbro; slfg—SLI ferrogabbro; dgh—diorite-granophyre hybrid; brd—Beaver River diabase. Locations of drill cores (DDH1, DDH2, DDH3) through sulfide horizon (narrow-spaced vertical lines)

Stop 2-4. Finland Granophyre

Location: Near site of Former USAF Radar Station Park near NAD83 UTM 15T 0633180E; 5256520N

Goals

  • Observe character of Finland Granophyre.

  • Discuss origin of Finland Granophyre and relationship to Sonju Lake Intrusion.

Description

The rocks at this stop are fairly typical of the upper part of the Finland Granophyre (Miller et al. 1993a, 1993b), a large (>40 km2) felsic body that structurally overlies the Sonju Lake Intrusion. The uppermost unit of the Finland granophyre (this stop) consists of homogeneous, micrographic leucogranite. Miarolitic cavities are common. Farther north, and at a structurally lower level, the leucogranites of the upper Finland Granophyre give way to coarse-grained and variably textured quartz ferromonzodiorites that are characterized by prismatic to subprismatic mafic silicates (e.g., ferrohedenbergite) and plagioclase set in a micrographic matrix.

The Finland Granophyre is similar to the other mainstage felsic intrusive and volcanic rocks that occur at higher structural levels within the northwestern Midcontinent Rift. Although felsic rocks formed during the early and main stages of rifting share similar major and trace element compositions, the felsic rocks of the main stage are characterized by lower Nb/Th and less radiogenic Nd isotopic compositions (Vervoort et al., 2007).

In the field, and in drill core, the contact relations between the Finland Granophyre and the Sonju Lake Intrusion are gradational and complex. The Finland Granophyre yielded a U-Pb zircon age of 1098.2 ± 5.5 Ma (Vervoort et al., 2007) and may be slightly older than the underlying Sonju Lake Intrusion (1096.1 ± 0.8 Ma; Paces and Miller, 1993), but additional high-precision dating is needed to confirm this. Rocks along the boundary between these two intrusive units exhibit complex interlayering of mafic and felsic rock types and varied textures. Whole-rock and mineral compositions vary gradually across the boundary zone. The subparallel trends of the two bodies and the compositional layering within them, and the lack of sharp chemical discontinuities along their contact zone, all seem to support a comagmatic relationship. In contrast, the large volume of granophyric rock, compared with the volume of mafic rocks of the Sonju Lake Intrusion, argues against a genetic relationship between the two units (Miller and Green, 2002).

Opportunities for Student Learning. The Finland Granophyre is challenging for students to work with in hand specimen and in the field. Outcrops are fairly homogeneous and unremarkable. The overall altered nature of the rock, and the lack of readily distinguishable quartz make it a challenge to classify in the field. In thin section, feldspars are relatively easily distinguished, however plagioclase is not always visibly twinned; granophyric intergrowths are well developed and present an opportunity to discuss rock texture and phase relations, alteration patterns in feldspars, and problems of classification.

Example Learning Objectives. Students will be able to:

  • interpret the texture and composition of the Finland Granophyre in terms of its map relations;

  • explain several methods that could be used to determine the age of this rock, and what each would tell you; and

  • propose models of the origin of felsic magmas in the Mid-continent Rift, and suggest types of evidence that could be used for testing those models.

NEXT: Return to the Twin Cities. Drive east on County Road 7 to MN Highway 1; then east on Highway 1 to MN-61S. Continue south MN-61S to I-35 S and to the Twin Cities.

References Cited

Ambrose
,
S.A.
Bridges
,
M.W.
DiPietro
,
M.
Lovett
,
M.C.
Norman
,
M.K.
,
2010
,
How learning works
:
Seven research-based principles for smart teaching
 :
Jossey Bass
,
301
p.
American Association for the Advancement of Science
,
1989
,
Science for All Americans
:
A Project 2061 report on literacy goals in science, mathematics, and technology
 :
Washington, D.C.
,
AAAS
, .
Boerboom
,
T.B.
Miller
,
J.D.
, Jr.
Green
,
J.C.
,
2004
,
Geologic highlights of new mapping in the southwestern sequence of the North Shore Volcanic Group and in the Beaver Bay Complex
:
Institute on Lake Superior Geology
,
50th Annual Meeting, Field Trip Guidebook, Proceedings
  Volume
50
,
Pt. II—Field Trip Guidebook
, p.
45
85
.
Boyle
,
A.
Maguire
,
S.
Martin
,
A.
Milsom
,
C.
Nash
,
R.
Rawlinson
,
S.
Turner
,
A.
Wurthmann
,
S.
Conchie
,
S.
,
2007
,
Fieldwork is good: The student perception and the affective domain
:
Journal of Geography in Higher Education
 , v.
31
, no.
2
, p.
299
317
, .
Bransford
,
J.D.
Brown
,
A.L.
Pellegrino
,
J.W.
, eds.,
2000
,
How People Learn: Brain, Mind
,
Experience and School
 :
Washington D.C.
,
National Academy Press
,
384
p.
Card
,
K.D.
,
1990
,
A review of the Superior Province of the Canadian Shield, a product of Archean accretion
:
Precambrian Research
 , v.
48
, p.
99
156
, .
Chandler
,
V.W.
,
2002
,
Geophysical characteristics of the Duluth Complex and associated rocks
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
52
75
.
Davis
,
D.W.
Green
,
J.C.
,
1997
,
Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution
:
Canadian Journal of Earth Sciences
 , v.
34
, p.
476
488
.
Fralick
,
P.
Davis
,
D.W.
Kissin
,
S.A.
,
2002
,
The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash
:
Canadian Journal of Earth Sciences
 , v.
39
, p.
1085
1091
, .
Green
,
J.C.
,
1989
,
Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota
:
Geological Society of America Bulletin
 , v.
101
, p.
486
500
, .
Green
,
J.C.
,
2002
,
Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeaster Minnesota
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
94
105
.
Green
,
J.C.
Fitz
,
T.J.
, III
,
1993
,
Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition
:
Journal of Volcanology and Geothermal Research
 , v.
54
, p.
177
196
, .
Green
,
J.C.
Bornhorst
,
T.J.
Chandler
,
V.W.
Mudrey
,
M.G.
, Jr.
Myers
,
P.E.
Pesonen
,
L.J.
Wilband
,
J.T.
,
1987
,
Keweenawan dykes of the Lake Superior region: Evidence for evolution of the Middle Proterozoic mid-continent rift system of North America
,
in
Halls
,
H.C.
Fahrig
,
W.F.
, eds.,
Mafic dyke swarms: Geological Association of Canada Special Paper 34
 , p.
289
302
.
Handelsman
,
J.
Miller
,
S.
Pfund
,
C.
,
2007
,
Scientific Teaching
 :
New York, New York
,
WH Freeman and Company
,
184
p.
Hoaglund
,
S.A.
,
2010
,
U-Pb geochronology of the Duluth Complex and related hypabyssal intrusions: investigating the emplacement history of a large multiphase intrusive complex related to the 1.1 Ga Midcontinent Rift
[
unpublished M.Sc. thesis
 ]:
University of Minnesota
,
Duluth
,
103
p.
Holm
,
D.K.
Schneider
,
D.A.
Rose
,
S.
Mancuso
,
C.
McKenzie
,
M.
Foland
,
K.A.
Hodges
,
K.V.
,
2007
,
Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution
:
Precambrian Research
 , v.
157
, p.
106
126
.
Jirsa
,
M.A.
,
1984
,
Interflow sedimentary rock in the Keweenawan North Shore Volcanic Group, northeastern Minnesota
:
Minnesota Geological Survey Report of Investigations 30
 ,
20
p.
Jirsa
,
M.A.
Boerboom
,
T.J.
Green
,
J.C.
Miller
,
J.D.
, Jr.
Morey
,
G.B.
Ojakangas
,
R.W.
Peterson
,
D.M.
,
2004
,
Classic outcrops of northeastern Minnesota
,
in
50th Annual Meeting of the Institute on Lake Superior Geology
 :
Proceedings Volume Part 2: Field Trip Guidebook
, v.
50
, p.
129
169
.
Manduca
,
C.
Carpenter
,
J.R.
,
2006
,
Introduction to special issue on “Teaching in the Field”
:
Journal of Geoscience Education
 , v.
54
, no.
2
, p.
92
.
Maskall
,
J.
Stokes
,
A.
,
2008
,
Designing Effective Fieldwork for the Environmental and Natural Sciences
:
GEES Subject Centre Teaching and Learning Guide
 ,
77
p., (
retrieved 31 May 2011
).
Miller
,
J.D.
, Jr.
, ed.,
1995a
,
Field trip guidebook for the geology and ore deposits of the Midcontinent Rift in the Lake Superior Region
:
Minnesota Geological Survey Guidebook Series
 , no.
20
,
216
p.
Miller
,
J.D.
, Jr.
,
1995b
,
Field Trip 3, Day 1: The Duluth Complex
,
in
Miller
,
J.D.
, Jr.
, ed.,
Field trip guidebook for the geology and ore deposits of the Midcontinent rift in the Lake Superior region: Minnesota Geological Survey Guidebook Series, no. 20
 , p.
123
148
.
Miller
,
J.D.
, Jr.
,
1995c
,
Field Trip 3, Day 2: The Beaver Bay Complex
,
in
Miller
,
J.D.
, Jr.
, ed.,
Field trip guidebook for the geology and ore deposits of the Midcontinent rift in the Lake Superior region: Minnesota Geological Survey Guidebook Series
 , no.
20
, p.
149
169
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
,
2002
,
Geology of the Beaver Bay Complex and related hypabyssal intrusions
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
144
163
.
Miller
,
J.D.
Green
,
J.C.
,
2008
,
Bedrock geology of the West Duluth and eastern portion of the Esko Quadrangles, St. Louis County, Minnesota
:
Minnesota Geological Survey Miscellaneous Map Series M-183
 ,
scale 1:24,000
.
Miller
,
J.D.
, Jr.
Severson
,
M.J.
,
2002
,
Geology of the Duluth Complex
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
,
2002
,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
106
163
.
Miller
,
J.D.
, Jr.
Vervoort
,
J.D.
,
1996
,
The latent magmatic stage of the Midcontinent rift: a period of magmatic underplating and melting of the lower crust
:
Institute on Lake Superior Geology, 42nd Annual Meeting, Proceedings: Geological Association of Canada
 ,
Program with Abstracts
, v.
42
, p.
33
35
.
Miller
,
J.D.
, Jr.
Weiblen
,
P.W.
,
1990
,
Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush
:
Journal of Petrology
 , v.
31
, p.
295
339
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Boerboom
,
T.B.
Chandler
,
V.W.
,
1993a
,
Geology of the Doyle Lake and Finland quadrangles, Lake County, Minnesota
:
Minnesota Geological Survey Miscellaneous Map Series, M-72
 ,
scale 1:24,000
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Boerboom
,
T.B.
Chandler
,
V.W.
,
1993b
,
Geology of the Illgen City quadrangle, Lake County, Minnesota
:
Minnesota Geological Survey Miscellaneous Map Series, M-69
 ,
scale 1:24,000
.
Miller
,
J.D.
, Jr.
Nicholson
,
S.W.
Cannon
,
W.F.
,
1995
,
The Midcontinent Rift in the Lake Superior region
in
Miller
,
J.D.
, Jr.
, ed.,
Field trip guidebook for the geology and ore deposits of the Midcontinent rift in the Lake Superior region: Minnesota Geological Survey Guidebook Series
 , no.
20
, p.
1
22
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Peterson
,
D.M.
,
2001
,
Geologic map of the Duluth Complex and related rocks, northeastern Minnesota
:
Minnesota Geological Survey Miscellaneous Map M-119
 ,
scale 1:200,000, 2 sheets
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
2002a
,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota
:
Minnesota Geological Survey Report of Investigations 58
 ,
207
p.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
,
2002b
,
Terminology, nomenclature and classification of Keweenawan igneous rocks of northeastern Minnesota
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
5
20
.
Miller
,
J.D.
, Jr.
Severson
,
M.J.
Hauck
,
S.A.
,
2002c
,
History of geologic mapping and mineral exploration in the Duluth Complex
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
21
51
.
Mogk
,
D.
,
2011
,
this volume, Get ready, get set, go…on a field trip
,
in
Miller
,
J.D.
Hudak
,
G.J.
Wittkop
,
C.
McLaughlin
,
P.I.
, eds.,
Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24
 , .
Morey
,
G.B.
Meints
,
J.
,
2000
,
Geologic Map of Minnesota: Bedrock Geology
:
Minnesota Geological Survey, State Map Series S-20
 , 3rd edition,
1:1,000,000
.
National Research Council
,
2005
,
How People Learn
:
Science in the Classroom
 :
Washington, D.C.
,
National Academies Press
,
264
p.
NICE (Northern Interior Continental Evolution) Working Group
,
Holm
,
D.K.
Anderson
,
R.
Boerboom
,
T.J.
Cannon
,
W.F.
Chandler
,
V.
Jirsa
,
M.
Miller
,
J.
Schneider
,
D.A.
Schulz
,
K.J.
Van Schmus
,
W.R.
,
2007
,
Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation
:
Precambrian Research
 , v.
157
, no.
1-4
, p.
71
79
.
Nicholson
,
S.W.
Shirey
,
S.B.
Schulz
,
K.J.
Green
,
J.C.
,
1997
,
Rift- wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development
:
Canadian Journal of Earth Sciences
 , v.
34
, p.
504
520
, .
Ojakangas
,
R.W.
Matsch
,
C.
,
1982
,
Minnesota’s Geology
 :
Minneapolis
,
University of Minnesota Press
,
225
p.
Ojakangas
,
R.W.
Severson
,
M.J.
Jongewaard
,
P.K.
Arola
,
J.L.
Evers
,
J.T.
Halverson
,
D.G.
Morey
,
G.B.
Holst
,
T.B.
,
2005
,
Geology and sedimentology of the Paleoproterozoic Animikie Group: The Pokegama Formation, The Biwabik Iron Formation, and Virginia Formation of the eastern Mesabi Iron Range, and the Thomson Formation ear Duluth, Northeastern Minnesota
,
in
Robinson
,
L.
, ed.,
Field trip guidebook for selected geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook 21
 , p.
208
237
.
Paces
,
J.B.
Miller
,
J.D.
, Jr.
,
1993
,
Precise U-Pb ages of Duluth Complex and related mafic intrusions of northeastern Minnesota: geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system
:
Journal of Geophysical Research
 , v.
98
, no.
B8
, p.
13,997
14,013
, .
Peterson
,
D.M.
Jirsa
,
M.A.
Hudak
,
G.J.
,
2009
,
Architecture of an Archean greenstone belt: stratigraphy, structure, and mineralization
,
in
55th Annual Meeting of the Institute on Lake Superior Geology
 :
Proceedings Volume Part 2, Field Trip Guidebook
, v.
55
, p.
179
215
.
Peterson
,
D.M.
Severson
,
M.J.
,
2002
,
Archean and Paleoproterozoic rocks that form the footwall of the Duluth Complex
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
76
93
.
Pound
,
K.
Campbell
,
K.
Schmitt
,
L.
,
2011
,
this volume, An Examination of the bedrock geology and the Mississippi River Valley in the Twin Cities: Pedagogical strategies for introductory geology field trips
,
in
Miller
,
J.D.
Hudak
,
G.J.
Wittkop
,
C.
McLaughlin
,
P.I.
, eds.,
Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America]: Geological Society of America Field Guide 24
 , .
Schmidt
,
S.T.
,
1990
,
Alteration under conditions of burial metamorphism in the North Shore Volcanic Group, Minnesota—Mineralogical and geochemical zonation
[
unpublished Ph.D. dissertation
 ]:
Heidelberg University
,
Germany
,
309
p.
Schmidt
,
S.Th.
,
1993
,
Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA
:
Journal of Metamorphic Geology
 , v.
11
, p.
401
414
, .
Schmidt
,
S.Th.
Green
,
J.C.
,
1992
,
Low grade metamorphism of the Keweenawan sequence in Minnesota and Michigan
:
International Geological Correlation Programme, Project 294, Very Low Grade Metamorphism, Guidebook for Post-meeting field trip
 ,
16–20 September 1992
,
77
p.
Schmidt
,
S.Th.
Robinson
,
D.
,
1997
,
Metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition of the North Shore Volcanic Group, Minnesota
:
Geological Society of America Bulletin
 , v.
109
, p.
683
697
, .
Severson
,
M.J.
Miller
,
J.D.
, Jr.
Peterson
,
D.M.
Green
,
J.C.
Hauck
,
S.A.
,
2002
,
Mineral potential of the Duluth Complex and related intrusions
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
164
200
.
Shirey
,
S.B.
Klewin
,
K.W.
Berg
,
J.H.
Carlson
,
R.W.
,
1994
,
Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keweenawan Mamainse Point Formation, Ontario, Canada
:
Geochimica et Cosmochimica Acta
 , v.
58
, p.
4475
4490
, .
Southwick
,
D.L.
Morey
,
G.B.
McSwiggen
,
P.L.
,
1988
,
Geologic map of the Penokean Orogen, central and eastern Minnesota, and accompanying text
:
Minnesota Geological Survey Report of Investigations 37
 ,
25
p.,
scale: 1:250,000
.
Stokes
,
A.
Boyle
,
A.P.
,
2009
,
The undergraduate geoscience fieldwork experience: Influencing factors and implications for learning
,
in
Whitmeyer
,
S.J.
Mogk
,
D.W.
Pyle
,
E.J.
, eds.,
Field Geology Education: Historical Perspectives and Modern Approaches: Geological Society of America Special Paper 461
 , p.
291
311
.
Vervoort
,
J.D.
Green
,
J.C.
,
1997
,
Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust
:
Canadian Journal of Earth Sciences
 , v.
34
, p.
521
535
, .
Vervoort
,
J.D.
Wirth
,
K.
Kennedy
,
B.
Sandland
,
T.
Harpp
,
K.
,
2007
,
The magmatic evolution of the Midcontinent Rift: New geochronologic and geochemical evidence from felsic magmatism
:
Precambrian Research
 , v.
157
, no.
1-4
, p.
235
268
, .
Whitmeyer
,
S.J.
Mogk
,
D.W.
Pyle
,
E.J.
, eds.,
2009
,
Field Geology Education: Historical Perspectives and Modern Approaches
:
Geological Society of America Special Paper 461
 ,
356
p.
Wiggins
,
G.
McTighe
,
J.
,
1998
,
Understanding by Design
:
Upper Saddle River
 ,
New Jersey
,
Association for Supervision and Curriculum Development, Prentice Hall
,
201
p.
Wirth
,
K.R.
Vervoort
,
J.D.
,
1995
,
Nd isotopic constraints on mantle and crustal contributions to Early Proterozoic dykes of the southern Superior Province
,
in
Baer
,
G.
Heimann
,
A.
, eds.,
Physics and Chemistry of Dykes
 :
Rotterdam
,
Balkema
, p.
237
249
.
Wirth
,
K.R.
Vervoort
,
J.
Craddock
,
J.P.
Davidson
,
C.
Finley-Blasi
,
L.
Kerber
,
L.
Lundquist
,
R.
Vorhies
,
S.
Walker
,
E.
,
2006
,
Source rock ages and patterns of sedimentation in the Lake Superior region: Results of preliminary U-Pb detrital zircon studies
:
Geological Society of America Abstracts with Programs
 , v.
38
, no.
7
, p.
505
.

Figures & Tables

Figure 1.

A design model for learning activities for fieldwork. From Stokes and Boyle (2009).

Figure 1.

A design model for learning activities for fieldwork. From Stokes and Boyle (2009).

Figure 2.

Superimposed magnetic on gravity (SMOG) map of Minnesota showing first vertical derivative of aero-magnetic data and raw gravity data for Minnesota. The derivative transformed magnetic data (shaded in gray) show relative magnetic susceptibility (lighter shades are more magnetic) and enhance the signatures of the near-surface bedrock, whereas the untransformed gravity data emphasizes sources (red shades are more dense; blue shades are less dense) deeper in the crust. Prepared by Val Chandler (Minnesota Geological Survey). Terrane boundaries from NICE Working Group (2007).

Figure 2.

Superimposed magnetic on gravity (SMOG) map of Minnesota showing first vertical derivative of aero-magnetic data and raw gravity data for Minnesota. The derivative transformed magnetic data (shaded in gray) show relative magnetic susceptibility (lighter shades are more magnetic) and enhance the signatures of the near-surface bedrock, whereas the untransformed gravity data emphasizes sources (red shades are more dense; blue shades are less dense) deeper in the crust. Prepared by Val Chandler (Minnesota Geological Survey). Terrane boundaries from NICE Working Group (2007).

Figure 3.

Generalized geologic map of Minnesota (after Morey and Meints, 2000).

Figure 3.

Generalized geologic map of Minnesota (after Morey and Meints, 2000).

Figure 4.

Generalized geologic map of the northeastern Minnesota. Modified from Miller et al. (2001).

Figure 4.

Generalized geologic map of the northeastern Minnesota. Modified from Miller et al. (2001).

Figure 5.

Volcanic stratigraphy of the North Shore Volcanic Group. Modified from Davis and Green (1997) by Hoaglund (2010).

Figure 5.

Volcanic stratigraphy of the North Shore Volcanic Group. Modified from Davis and Green (1997) by Hoaglund (2010).

Figure 6.

Magmatic stages of Midcontinent Rift evolution. Modified from Vervoort et al. (2007) and Miller and Vervoort (1996).

Figure 6.

Magmatic stages of Midcontinent Rift evolution. Modified from Vervoort et al. (2007) and Miller and Vervoort (1996).

Figure 7.

Idealized internal structure of olivine tholeiite-composition lava flows. From Green (1989).

Figure 7.

Idealized internal structure of olivine tholeiite-composition lava flows. From Green (1989).

Figure 8.

Traverse map for lower Sonju Lake Intrusion. Modified from Miller et al. (1993a). Unit symbols (oldest to youngest): og—early olivine gabbro or Beaver Bay Complex; bld—Blessner Lake diabase; sldp—SLI diabasic picrite; sld—SLI dunite; slt—SLI troctolite; slg—SLI gabbro; slfg—SLI ferrogabbro; dgh—diorite-granophyre hybrid; brd—Beaver River diabase.

Figure 8.

Traverse map for lower Sonju Lake Intrusion. Modified from Miller et al. (1993a). Unit symbols (oldest to youngest): og—early olivine gabbro or Beaver Bay Complex; bld—Blessner Lake diabase; sldp—SLI diabasic picrite; sld—SLI dunite; slt—SLI troctolite; slg—SLI gabbro; slfg—SLI ferrogabbro; dgh—diorite-granophyre hybrid; brd—Beaver River diabase.

Figure 9.

Traverse map through upper part of Sonju Lake Intrusion and lower Finland Granophyre (Miller, 1995c). Route follows unnamed logging road. Outcrops along road are indicated with solid black fill. Units (oldest to youngest): frpm—quartz ferromonzodiorite (Finland Granophyre); frg—leucogranite (Finland Granophyre); slt—SLI troctolite; slg—SLI gabbro; slfg—SLI ferrogabbro; dgh—diorite-granophyre hybrid; brd—Beaver River diabase. Locations of drill cores (DDH1, DDH2, DDH3) through sulfide horizon (narrow-spaced vertical lines)

Figure 9.

Traverse map through upper part of Sonju Lake Intrusion and lower Finland Granophyre (Miller, 1995c). Route follows unnamed logging road. Outcrops along road are indicated with solid black fill. Units (oldest to youngest): frpm—quartz ferromonzodiorite (Finland Granophyre); frg—leucogranite (Finland Granophyre); slt—SLI troctolite; slg—SLI gabbro; slfg—SLI ferrogabbro; dgh—diorite-granophyre hybrid; brd—Beaver River diabase. Locations of drill cores (DDH1, DDH2, DDH3) through sulfide horizon (narrow-spaced vertical lines)

Contents

References

References Cited

Ambrose
,
S.A.
Bridges
,
M.W.
DiPietro
,
M.
Lovett
,
M.C.
Norman
,
M.K.
,
2010
,
How learning works
:
Seven research-based principles for smart teaching
 :
Jossey Bass
,
301
p.
American Association for the Advancement of Science
,
1989
,
Science for All Americans
:
A Project 2061 report on literacy goals in science, mathematics, and technology
 :
Washington, D.C.
,
AAAS
, .
Boerboom
,
T.B.
Miller
,
J.D.
, Jr.
Green
,
J.C.
,
2004
,
Geologic highlights of new mapping in the southwestern sequence of the North Shore Volcanic Group and in the Beaver Bay Complex
:
Institute on Lake Superior Geology
,
50th Annual Meeting, Field Trip Guidebook, Proceedings
  Volume
50
,
Pt. II—Field Trip Guidebook
, p.
45
85
.
Boyle
,
A.
Maguire
,
S.
Martin
,
A.
Milsom
,
C.
Nash
,
R.
Rawlinson
,
S.
Turner
,
A.
Wurthmann
,
S.
Conchie
,
S.
,
2007
,
Fieldwork is good: The student perception and the affective domain
:
Journal of Geography in Higher Education
 , v.
31
, no.
2
, p.
299
317
, .
Bransford
,
J.D.
Brown
,
A.L.
Pellegrino
,
J.W.
, eds.,
2000
,
How People Learn: Brain, Mind
,
Experience and School
 :
Washington D.C.
,
National Academy Press
,
384
p.
Card
,
K.D.
,
1990
,
A review of the Superior Province of the Canadian Shield, a product of Archean accretion
:
Precambrian Research
 , v.
48
, p.
99
156
, .
Chandler
,
V.W.
,
2002
,
Geophysical characteristics of the Duluth Complex and associated rocks
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
52
75
.
Davis
,
D.W.
Green
,
J.C.
,
1997
,
Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution
:
Canadian Journal of Earth Sciences
 , v.
34
, p.
476
488
.
Fralick
,
P.
Davis
,
D.W.
Kissin
,
S.A.
,
2002
,
The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash
:
Canadian Journal of Earth Sciences
 , v.
39
, p.
1085
1091
, .
Green
,
J.C.
,
1989
,
Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota
:
Geological Society of America Bulletin
 , v.
101
, p.
486
500
, .
Green
,
J.C.
,
2002
,
Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeaster Minnesota
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
94
105
.
Green
,
J.C.
Fitz
,
T.J.
, III
,
1993
,
Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition
:
Journal of Volcanology and Geothermal Research
 , v.
54
, p.
177
196
, .
Green
,
J.C.
Bornhorst
,
T.J.
Chandler
,
V.W.
Mudrey
,
M.G.
, Jr.
Myers
,
P.E.
Pesonen
,
L.J.
Wilband
,
J.T.
,
1987
,
Keweenawan dykes of the Lake Superior region: Evidence for evolution of the Middle Proterozoic mid-continent rift system of North America
,
in
Halls
,
H.C.
Fahrig
,
W.F.
, eds.,
Mafic dyke swarms: Geological Association of Canada Special Paper 34
 , p.
289
302
.
Handelsman
,
J.
Miller
,
S.
Pfund
,
C.
,
2007
,
Scientific Teaching
 :
New York, New York
,
WH Freeman and Company
,
184
p.
Hoaglund
,
S.A.
,
2010
,
U-Pb geochronology of the Duluth Complex and related hypabyssal intrusions: investigating the emplacement history of a large multiphase intrusive complex related to the 1.1 Ga Midcontinent Rift
[
unpublished M.Sc. thesis
 ]:
University of Minnesota
,
Duluth
,
103
p.
Holm
,
D.K.
Schneider
,
D.A.
Rose
,
S.
Mancuso
,
C.
McKenzie
,
M.
Foland
,
K.A.
Hodges
,
K.V.
,
2007
,
Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution
:
Precambrian Research
 , v.
157
, p.
106
126
.
Jirsa
,
M.A.
,
1984
,
Interflow sedimentary rock in the Keweenawan North Shore Volcanic Group, northeastern Minnesota
:
Minnesota Geological Survey Report of Investigations 30
 ,
20
p.
Jirsa
,
M.A.
Boerboom
,
T.J.
Green
,
J.C.
Miller
,
J.D.
, Jr.
Morey
,
G.B.
Ojakangas
,
R.W.
Peterson
,
D.M.
,
2004
,
Classic outcrops of northeastern Minnesota
,
in
50th Annual Meeting of the Institute on Lake Superior Geology
 :
Proceedings Volume Part 2: Field Trip Guidebook
, v.
50
, p.
129
169
.
Manduca
,
C.
Carpenter
,
J.R.
,
2006
,
Introduction to special issue on “Teaching in the Field”
:
Journal of Geoscience Education
 , v.
54
, no.
2
, p.
92
.
Maskall
,
J.
Stokes
,
A.
,
2008
,
Designing Effective Fieldwork for the Environmental and Natural Sciences
:
GEES Subject Centre Teaching and Learning Guide
 ,
77
p., (
retrieved 31 May 2011
).
Miller
,
J.D.
, Jr.
, ed.,
1995a
,
Field trip guidebook for the geology and ore deposits of the Midcontinent Rift in the Lake Superior Region
:
Minnesota Geological Survey Guidebook Series
 , no.
20
,
216
p.
Miller
,
J.D.
, Jr.
,
1995b
,
Field Trip 3, Day 1: The Duluth Complex
,
in
Miller
,
J.D.
, Jr.
, ed.,
Field trip guidebook for the geology and ore deposits of the Midcontinent rift in the Lake Superior region: Minnesota Geological Survey Guidebook Series, no. 20
 , p.
123
148
.
Miller
,
J.D.
, Jr.
,
1995c
,
Field Trip 3, Day 2: The Beaver Bay Complex
,
in
Miller
,
J.D.
, Jr.
, ed.,
Field trip guidebook for the geology and ore deposits of the Midcontinent rift in the Lake Superior region: Minnesota Geological Survey Guidebook Series
 , no.
20
, p.
149
169
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
,
2002
,
Geology of the Beaver Bay Complex and related hypabyssal intrusions
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
144
163
.
Miller
,
J.D.
Green
,
J.C.
,
2008
,
Bedrock geology of the West Duluth and eastern portion of the Esko Quadrangles, St. Louis County, Minnesota
:
Minnesota Geological Survey Miscellaneous Map Series M-183
 ,
scale 1:24,000
.
Miller
,
J.D.
, Jr.
Severson
,
M.J.
,
2002
,
Geology of the Duluth Complex
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
,
2002
,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
106
163
.
Miller
,
J.D.
, Jr.
Vervoort
,
J.D.
,
1996
,
The latent magmatic stage of the Midcontinent rift: a period of magmatic underplating and melting of the lower crust
:
Institute on Lake Superior Geology, 42nd Annual Meeting, Proceedings: Geological Association of Canada
 ,
Program with Abstracts
, v.
42
, p.
33
35
.
Miller
,
J.D.
, Jr.
Weiblen
,
P.W.
,
1990
,
Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush
:
Journal of Petrology
 , v.
31
, p.
295
339
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Boerboom
,
T.B.
Chandler
,
V.W.
,
1993a
,
Geology of the Doyle Lake and Finland quadrangles, Lake County, Minnesota
:
Minnesota Geological Survey Miscellaneous Map Series, M-72
 ,
scale 1:24,000
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Boerboom
,
T.B.
Chandler
,
V.W.
,
1993b
,
Geology of the Illgen City quadrangle, Lake County, Minnesota
:
Minnesota Geological Survey Miscellaneous Map Series, M-69
 ,
scale 1:24,000
.
Miller
,
J.D.
, Jr.
Nicholson
,
S.W.
Cannon
,
W.F.
,
1995
,
The Midcontinent Rift in the Lake Superior region
in
Miller
,
J.D.
, Jr.
, ed.,
Field trip guidebook for the geology and ore deposits of the Midcontinent rift in the Lake Superior region: Minnesota Geological Survey Guidebook Series
 , no.
20
, p.
1
22
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Peterson
,
D.M.
,
2001
,
Geologic map of the Duluth Complex and related rocks, northeastern Minnesota
:
Minnesota Geological Survey Miscellaneous Map M-119
 ,
scale 1:200,000, 2 sheets
.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
2002a
,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota
:
Minnesota Geological Survey Report of Investigations 58
 ,
207
p.
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
,
2002b
,
Terminology, nomenclature and classification of Keweenawan igneous rocks of northeastern Minnesota
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
5
20
.
Miller
,
J.D.
, Jr.
Severson
,
M.J.
Hauck
,
S.A.
,
2002c
,
History of geologic mapping and mineral exploration in the Duluth Complex
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
21
51
.
Mogk
,
D.
,
2011
,
this volume, Get ready, get set, go…on a field trip
,
in
Miller
,
J.D.
Hudak
,
G.J.
Wittkop
,
C.
McLaughlin
,
P.I.
, eds.,
Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24
 , .
Morey
,
G.B.
Meints
,
J.
,
2000
,
Geologic Map of Minnesota: Bedrock Geology
:
Minnesota Geological Survey, State Map Series S-20
 , 3rd edition,
1:1,000,000
.
National Research Council
,
2005
,
How People Learn
:
Science in the Classroom
 :
Washington, D.C.
,
National Academies Press
,
264
p.
NICE (Northern Interior Continental Evolution) Working Group
,
Holm
,
D.K.
Anderson
,
R.
Boerboom
,
T.J.
Cannon
,
W.F.
Chandler
,
V.
Jirsa
,
M.
Miller
,
J.
Schneider
,
D.A.
Schulz
,
K.J.
Van Schmus
,
W.R.
,
2007
,
Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation
:
Precambrian Research
 , v.
157
, no.
1-4
, p.
71
79
.
Nicholson
,
S.W.
Shirey
,
S.B.
Schulz
,
K.J.
Green
,
J.C.
,
1997
,
Rift- wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development
:
Canadian Journal of Earth Sciences
 , v.
34
, p.
504
520
, .
Ojakangas
,
R.W.
Matsch
,
C.
,
1982
,
Minnesota’s Geology
 :
Minneapolis
,
University of Minnesota Press
,
225
p.
Ojakangas
,
R.W.
Severson
,
M.J.
Jongewaard
,
P.K.
Arola
,
J.L.
Evers
,
J.T.
Halverson
,
D.G.
Morey
,
G.B.
Holst
,
T.B.
,
2005
,
Geology and sedimentology of the Paleoproterozoic Animikie Group: The Pokegama Formation, The Biwabik Iron Formation, and Virginia Formation of the eastern Mesabi Iron Range, and the Thomson Formation ear Duluth, Northeastern Minnesota
,
in
Robinson
,
L.
, ed.,
Field trip guidebook for selected geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook 21
 , p.
208
237
.
Paces
,
J.B.
Miller
,
J.D.
, Jr.
,
1993
,
Precise U-Pb ages of Duluth Complex and related mafic intrusions of northeastern Minnesota: geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system
:
Journal of Geophysical Research
 , v.
98
, no.
B8
, p.
13,997
14,013
, .
Peterson
,
D.M.
Jirsa
,
M.A.
Hudak
,
G.J.
,
2009
,
Architecture of an Archean greenstone belt: stratigraphy, structure, and mineralization
,
in
55th Annual Meeting of the Institute on Lake Superior Geology
 :
Proceedings Volume Part 2, Field Trip Guidebook
, v.
55
, p.
179
215
.
Peterson
,
D.M.
Severson
,
M.J.
,
2002
,
Archean and Paleoproterozoic rocks that form the footwall of the Duluth Complex
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
76
93
.
Pound
,
K.
Campbell
,
K.
Schmitt
,
L.
,
2011
,
this volume, An Examination of the bedrock geology and the Mississippi River Valley in the Twin Cities: Pedagogical strategies for introductory geology field trips
,
in
Miller
,
J.D.
Hudak
,
G.J.
Wittkop
,
C.
McLaughlin
,
P.I.
, eds.,
Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America]: Geological Society of America Field Guide 24
 , .
Schmidt
,
S.T.
,
1990
,
Alteration under conditions of burial metamorphism in the North Shore Volcanic Group, Minnesota—Mineralogical and geochemical zonation
[
unpublished Ph.D. dissertation
 ]:
Heidelberg University
,
Germany
,
309
p.
Schmidt
,
S.Th.
,
1993
,
Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA
:
Journal of Metamorphic Geology
 , v.
11
, p.
401
414
, .
Schmidt
,
S.Th.
Green
,
J.C.
,
1992
,
Low grade metamorphism of the Keweenawan sequence in Minnesota and Michigan
:
International Geological Correlation Programme, Project 294, Very Low Grade Metamorphism, Guidebook for Post-meeting field trip
 ,
16–20 September 1992
,
77
p.
Schmidt
,
S.Th.
Robinson
,
D.
,
1997
,
Metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition of the North Shore Volcanic Group, Minnesota
:
Geological Society of America Bulletin
 , v.
109
, p.
683
697
, .
Severson
,
M.J.
Miller
,
J.D.
, Jr.
Peterson
,
D.M.
Green
,
J.C.
Hauck
,
S.A.
,
2002
,
Mineral potential of the Duluth Complex and related intrusions
,
in
Miller
,
J.D.
, Jr.
Green
,
J.C.
Severson
,
M.J.
Chandler
,
V.W.
Hauck
,
S.A.
Peterson
,
D.M.
Wahl
,
T.E.
, eds.,
Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58
 , p.
164
200
.
Shirey
,
S.B.
Klewin
,
K.W.
Berg
,
J.H.
Carlson
,
R.W.
,
1994
,
Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keweenawan Mamainse Point Formation, Ontario, Canada
:
Geochimica et Cosmochimica Acta
 , v.
58
, p.
4475
4490
, .
Southwick
,
D.L.
Morey
,
G.B.
McSwiggen
,
P.L.
,
1988
,
Geologic map of the Penokean Orogen, central and eastern Minnesota, and accompanying text
:
Minnesota Geological Survey Report of Investigations 37
 ,
25
p.,
scale: 1:250,000
.
Stokes
,
A.
Boyle
,
A.P.
,
2009
,
The undergraduate geoscience fieldwork experience: Influencing factors and implications for learning
,
in
Whitmeyer
,
S.J.
Mogk
,
D.W.
Pyle
,
E.J.
, eds.,
Field Geology Education: Historical Perspectives and Modern Approaches: Geological Society of America Special Paper 461
 , p.
291
311
.
Vervoort
,
J.D.
Green
,
J.C.
,
1997
,
Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust
:
Canadian Journal of Earth Sciences
 , v.
34
, p.
521
535
, .
Vervoort
,
J.D.
Wirth
,
K.
Kennedy
,
B.
Sandland
,
T.
Harpp
,
K.
,
2007
,
The magmatic evolution of the Midcontinent Rift: New geochronologic and geochemical evidence from felsic magmatism
:
Precambrian Research
 , v.
157
, no.
1-4
, p.
235
268
, .
Whitmeyer
,
S.J.
Mogk
,
D.W.
Pyle
,
E.J.
, eds.,
2009
,
Field Geology Education: Historical Perspectives and Modern Approaches
:
Geological Society of America Special Paper 461
 ,
356
p.
Wiggins
,
G.
McTighe
,
J.
,
1998
,
Understanding by Design
:
Upper Saddle River
 ,
New Jersey
,
Association for Supervision and Curriculum Development, Prentice Hall
,
201
p.
Wirth
,
K.R.
Vervoort
,
J.D.
,
1995
,
Nd isotopic constraints on mantle and crustal contributions to Early Proterozoic dykes of the southern Superior Province
,
in
Baer
,
G.
Heimann
,
A.
, eds.,
Physics and Chemistry of Dykes
 :
Rotterdam
,
Balkema
, p.
237
249
.
Wirth
,
K.R.
Vervoort
,
J.
Craddock
,
J.P.
Davidson
,
C.
Finley-Blasi
,
L.
Kerber
,
L.
Lundquist
,
R.
Vorhies
,
S.
Walker
,
E.
,
2006
,
Source rock ages and patterns of sedimentation in the Lake Superior region: Results of preliminary U-Pb detrital zircon studies
:
Geological Society of America Abstracts with Programs
 , v.
38
, no.
7
, p.
505
.

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