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Abstract

This field trip examines deformed Archean basement, and variably metamorphosed supracrustal rocks and an exhumed midcrustal batholith of late Paleoproterozoic age in east-central Minnesota. Collectively, these rocks reveal an approximately 100 m.y. geologic history of crustal growth and stabilization of this part of the craton. The Penokean orogen in Minnesota consists of a northern foreland basin (the Animikie basin), a medial fold-thrust belt, and a southern high-grade metamorphic and plutonic terrane, representing two major orogenic events: the Penokean (geon 18) and Yavapai (geon 17) orogenies. The 1870–1830 Ma Penokean orogenic rocks are part of a belt of juvenile crust accreted onto the southern margin of Laurentia-Baltic continent during the late Paleoproterozoic. Metamorphism along the southern margin of the Archean Superior province has been historically attributed to the Penokean Orogeny, in a corridor of amphibolite-facies rocks which record 1.86–1.80 Ga (geon 18) metamorphic ages that correspond to the culmination of arc accretion. However, a widespread geon 17 amphibolite-facies metamorphic overprint is also recorded along the regions of greatest thickening of the Penokean crust, which corresponds to the tectonically buried Archean-Proterozoic continental margin. This was also the locus of emplacement of the voluminous east-central Minnesota batholith, composed of some twenty separate intrusions that range from mafic to dominantly felsic-intermediate compositions. Most of these are Yavapai in age, with emplacement ages between 1787 and 1772 Ma.

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