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High-precision U-Pb geochronology in the Minnesota River Valley subprovince and its bearing on the Neoarchean to Paleoproterozoic evolution of the southern Superior Province

M.D. Schmitz^,1, S.A. Bowring^,2, D.L. Southwick^,3, T.J. Boerboom^#,3 and K.R. Wirth^,4

¹ Department of Geosciences, Boise State University, Boise, Idaho 83725, USA
² Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
³ Minnesota Geological Survey, University of Minnesota, St. Paul, Minnesota 55114, USA
⁴ Department of Geology, Macalester College, St. Paul, Minnesota 55105, USA

High-precision U-Pb ages have been obtained for high-grade gneisses, late-kinematic to postkinematic granitic plutons, and a crosscutting mafic dike of the Archean Minnesota River Valley tectonic subprovince, at the southern ramparts of the Superior craton of North America. The antiquity of the Minnesota River Valley terranes is confirmed by a high-precision U-Pb zircon age of 3422 ± 2 Ma for a tonalitic phase of the Morton Gneiss. Voluminous, late-kinematic monzogranites of the Benson (Ortonville granite) and Morton (Sacred Heart granite) blocks yield identical crystallization ages of 2603 ± 1 Ma, illustrating the synchrony and rapidity of deep crustal melting and plutonism throughout the Minnesota River Valley terranes. Postkinematic, 2591 ± 2 Ma syenogranites and aplitic dikes in both blocks effectively constrain the final penetrative deformation of the Minnesota River Valley subprovince. Monazite growth from 2609 to 2595 Ma in granulitic paragneisses of the Benson and Montevideo blocks is interpreted to record prograde to peak granulite facies metamorphic conditions associated with crustal thickening and magmatism. Neoarchean metamorphism and plutonism are interpreted to record the timing of collisional accretion and terminal suturing of the Mesoarchean continental Minnesota River Valley terranes to the southern margin of the Superior Province, along the western Great Lakes tectonic zone. Subsequent Paleoproterozoic rifting of this margin is recorded by voluminous basaltic dike intrusion, expressed in the Minnesota River Valley by major WNW-trending tholeiitic diabase dikes dated at 2067 ± 1 Ma, only slightly younger than the structurally and geochemically similar 2077 ± 4 Ma Fort Frances (Kenora-Kabetogama) dike swarm of northern Minnesota and adjoining Canada.

Key Words: Minnesota River Valley • Superior Province • U-Pb • zircon • monazite • granites • collisional orogenesis • Archean • cratons

This article has been cited by other articles:

				M.E. Bickford, J.L. Wooden, and R.L. Bauer 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, January 1, 2006; 118(1-2): 94 - 108. [Abstract] [Full Text] [PDF]