Chapter 6.1 Paleoarchean Gneisses in the Minnesota River Valley and Northern Michigan, USA
Publisher Summary This chapter elaborates the Paleoarchean gneisses in the Minnesota river valley and northern Michigan, USA. Meso- to Paleoarchean gneisses occur along the southern margin of the Neoarchean Superior Craton. The most extensive exposure of these rocks is in the Minnesota River Valley (MRV) of southwestern Minnesota, but there are also exposures in northern Michigan. Aeromagnetic mapping of southwestern Minnesota, and detailed gravity and magnetic modeling within the MRV have delineated four crustal blocks in the MRV that are bounded by three east-northeast-trending geophysical anomalies that roughly parallel the Morris fault segment. Most of the work in the MRV has concentrated on exposures in the Montevideo block and the Morton block, which are separated by the Yellow Medicine shear zone. The data indicate that major common tectonothermal events, recognized in the zircon geochronology, occurred in both the Morton and the Montevideo blocks. A mafic intrusion in the granite gneiss at Granite Falls has a well-constrained age of 3140 Ma, indicating another intrusive magmatic event at that time, and zircon overgrowths of this age are also seen in zircons from the Morton Gneiss.