precambrian evolution of minnesota

JOHN CRADDOCK, Macalester CollegeKARL WIRTH, Macalester College

CAMERON DAVIDSON, Carleton College

Craddock et al., 2006. 19th Annual Keck Symposium; http://keck.wooster.edu/publications

PRECAMBRIAN EVOLUTION OF MINNESOTA

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PROJECT SUMMARY

This project was organized to address outstanding geologic problems in the mid-continent region (Minnesota, Wisconsin and Michigan) using Macalester College as a base for field and lab studies June 14-July15, 2005. Following a regional field trip through the geology of northern Minnesota, students selected projects on igneous geochemistry, magnetic fabrics in igneous rocks, the provenance of clastic sedimentary rocks using detrital zircons, or a combination of the above. Following individual sample collection (Figs. 1 & 2), all students processed 2-3 clastic sediments for their zircon populations at Macalester. All students also used anisotropy of magnetic susceptibility (AMS) to measure strain (sandstones or quartzites) or a magmatic fabric (rhyolites, granites, diabase) at the Institute for Rock Magnetism (IRM) at the University of Minnesota. Magnetic separation of zircons continued in the fall at Macalester, and Prof. Davidson traveled to Pullman in December, 2005 to mount, polish, and scan (SEM-EDS and CL images) the pucks for laser analysis. Five students traveled to Washington State University, Pullman, WA in January, 2006 with Profs. Wirth and Craddock to use the laser ablation-mass spectrometer (LA-ICPMS) system to obtain U-Pb ages on igneous and detrital zircon populations. One granite (n=20 zircons) and sixteen sedimentary units (n=2000 zircons) were analyzed over ~3 weeks of lab time in Pullman.

Keweenaw Rift Rhyolite (Dhiren Patel, Ryan Porter)

The Keweenaw rift province was magmatically active for a brief period (1.1-1.06 Ga; Davis and Green, 1997) as a back-arc continental spreading center in-board of the convergent Grenville orogen (1.4-0.9 Ga). The rift produced a thick sequence of intrusive and extrusive igneous rocks, dominated by basalt (~70%) and rhyolite (~15%) and lesser bi-modal components including gabbro, granophyre, anorthosite, and lamprophyre. The rhyolites are found near the top of the igneous stratigraphy and are <30 m in thickness; some preserve no obvious layering while other flows contain contorted flow folding at the base and tops. Dhiren Patel and Ryan Porter selected four flows (Figs. 1 & 2) where oriented samples could be collected from the base, middle and top of a flow and ~20 oriented equant cores (or cubes) could be extracted for magnetic analysis.

Anisotropy of magnetic susceptibility (AMS) is a technique used to measure a magnetic fabric ellipsoid as a proxy for strain (sediments) or magmatic flow. Patel and Porter hypothesized that the tops and bottoms of flows would preserve a chaotic (random) AMS fabric and the flow interiors would preserve a magnetic fabric with K

max parallel to layering and trending

NW-SE, normal to the rift axis. In thin section, some of the rhyolites contain glass shards and tridymite pseudomorphs that are aligned parallel to bedding and normal to the rift. The petrofabric and AMS results for the rhyolite

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Craddock et al.,2006. 19th Annual Keck Symposium; http://keck.wooster.edu/publications

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Figure 1: Geologic map of the upper midwest including parts of Minnesota, Wisconsin, and Michigan (modified from Cannon et al., 1997).

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flows suggest the rhyolite eruptive direction was from the rift axis to the northwest.

Keweenaw Rift Granites (Natalie Juda, Matt Andring)

The central portion of the rift, exposed along the shore of Lake Superior in northeast Minnesota, is defined by the chaotic Beaver Bay igneous complex (Fig. 1). Diabase is the dominant igneous rock that surrounds a host of granitic and anorthositic xenoliths intruded by various mafic dike swarms. We chose two end-member field relations, 1) a granitic xenolith (1 square km), cross-cut by a mafic dike surrounded by diabase (Natalie Juda) and 2) diabase surrounding and surrounded by a swarm of “circular” granitic dikes (Matt Andring).

Juda showed that the granite xenolith is the by-product of rift magmatism, with a U-Pb zircon age of 1094 ± 11 Ma, and not a floated piece of Archean crust. The major and trace element geochemistry is similar to other rift granites and granophyres, and the lack of an AMS fabric in the host diabase, granite or basaltic dike is representative of the chaos of the intrusion of the Beaver Bay complex for a within-plate, low-potassium granite. Andring determined that the radial granite dikes and adjacent diabase are also geochemically similar to other rift granites and granophyres and the regional diabase composition. The AMS results for the granite dikes and diabase were also inconclusive, preserving random magmatic flow during intrusion and crystallization.

Minnesota Wisconsin-Michigan

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Archean Basement (Gneisses 3.6-3.0 Ga; Granite-Greenstone Terranes 2.7 Ga)

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Rove Formation (LK) Thomson Formation (LK) Tyler Formation (BL)

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Figure 2: Generalized regional stratigraphic correlation diagram for the mid-continent region in Minnesota and Wisconsin. Student participants are identified by their initials next to their sample interval. Yellow stars indicate a detrital zircon sample completed by a student whereas white stars indicate a clastic unit that was processed for zircons but not reported here.

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Detrital Zircon Ages and Provenance (Lee Finley-Blasi, Erin Walker, Laura Kerber, Becky Lundquist, Sarah Vorhies)

The southern Canadian Shield is characterized by a series of NE-trending terranes with radiometric ages of ~2.7 Ga and a few protocontinental terranes with older (3.0-3.6 Ga) gneisses (Southwick and Chandler, 1996; Bickford et al., 2006). Deposited on, and accreted to, the Archean craton are the foreland sediments and thrust sheets of the 1.8 Ga Penokean orogen, followed by the Yavapai (1.75 Ga), Mazatzal (1.65 Ga), and Grenville-Keweenaw (1.4-0.9 Ga) orogens (Karlstrom et al., 1999; Holm et al., 2005). We chose to address the age and provenance of clastic sedimentary units in the adjacent and overlapping Penokean-Animikie and younger Keweenaw sequence where the regional stratigraphy is well-constrained but little is known of the absolute depositional ages or provenance (Fig. 2; Ojakangas et al., 2001).

Lee Finley-Blasi studied the post-Keweenaw Fond du Lac and overlying Hinckley Sandstones. The older Fond du Lac Formation (youngest grain: 1010 Ma) contains a significant population of pre-rift zircons, presumably transported from the Grenville orogen to the east, whereas the younger Hinckley sands (youngest grain: 1052 Ma) contain many older, and locally-derived zircons. At the base of the Keweenaw rift lies the Nopeming (Duluth), Puckwunge (N. Minnesota) and Bessemer (east side of the rift) Sandstones.

Erin Walker analyzed the zircon populations of the Nopeming and Puckwunge Formations, finding the former to contain a concentration of pre-rift (1.2 Ga) zircons, Penokean-aged zircons (~1.8 Ga) and a group from 2.2-2.6 Ga. The Puckwunge contains many older, Archean populations, but surprisingly no zircons younger than Penokean.

Underlying the rift is the Pre-Penokean Animike basin sequence with the correlative Rove (northern MN), Thomson (Duluth) and Tyler (east, MI) Formations composed of interbedded sand and shale (turbidites). Laura Kerber studied the Thomson and Rove Formations, both of which contain predominant zircon populations of ~1850-2000 Ma and a variety of ages back to 3.5 Ga. The maximum depositional age of the Thomson is 1841 Ma, and 1826 Ma for the Rove.

The Tyler Formation, east of the Keweenaw rift, was studied by Becky Lundquist, has a maximum depositional age of ~1818 Ma, and its zircon population is dominated by Penokean ages and rocks from the underlying 2.7 Ga granite-greenstone belts. Lundquist also analyzed the Palms Formation at the base of the Marquette Supergroup, which contains zircons dating 2.7 Ga, a few at 3.4 Ga and the youngest zircon is 2594 Ma.

In east-central Minnesota, overlying the Archean McGrath gneiss is the locally-exposed and enigmatic Denham Formation (Boerboom and Jirsa, 2001), which was deformed and metamorphosed to amphibolite facies during the Penokean orogeny (Holm and Selverstone, 1990). Sarah Vorhies analyzed the basal, metamorphosed arkosic sandstone and an upper arkose, finding zircon populations between 2.1-3.4 Ga and 2.7-3.4 Ga, respectively. Pillow basalts from the middle Denham Formation yield an alkaline, rift-related geochemical signature.

REFERENCESBickford, M.E., Wooden, J.L., and Bauer, R.L., 2006,

SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley: Implications for the tectonic history of the Superior Province: Geological Society of America Bulletin, v. 118, no. 1/2, p. 94-108.

Boerboom , T. J. and Jirsa, Mark A, 2001, Stratigraphy

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Murphy, J. 2006. 19th Annual Keck Symposium; http://keck.wooster.edu/publications

of the Paleoproterozoic Denham Formation; a continental margin assemblage of basalt, arkose, and dolomite:47th annual meeting, Institute on Lake Superior Geology, p. 6-7.

Cannon, W.F., Kress, T.H., Sutphin, D.M., Morey, G.B., Meints, J., and Barber-Delach, R., 1999, Deposits of the Lake Superior region: U.S. Geological Survey Open-File Report 97-0455.

Davis, D. W., and 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, no. 4, p. 476-488.

Holm, D., and Selverstone, J., 1990, Rapid growth and strain rates inferred from synkinematic garnets, Penokean orogen, Minnesota: Geology, v. 18, p. 166–169, doi:10.1130/0091-7613(1990)0182.3.CO;2.

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precambrian evolution of minnesota

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