The Compeau Creek Gneiss is a good example of Archean-aged basement rock in the Marquette area of Michigan's Upper Peninsula (UP).  It underlies metamorphosed sedimentary rocks (quartzites, metadolostones, slates, iron-formation) of the Marquette Range Supergroup.  The Compeau Creek is a coarsely-crystalline granitic gneiss, formed by high-grade regional metamorphism of intrusive felsic/intermediate igneous rock, likely in an ancient island arc setting.  This interpretation is based on consideration of other basement lithologies in this region of the UP, such as pillow basalts, volcanic breccias, and serpentinized peridotites. 


The protolith (original rock before metamorphism) of the Compeau Creek Gneiss has been inferred to be tonalite or granodiorite of Neoarchean age (apparently somewhere between 2.7 and 3.0 billion years).


Compeau Creek Gneiss sample (~7.7 cm across at its widest) of Archean age from a Co. Rd. 553 roadcut south of Marquette, UP of Michigan, USA (see map).  Reddish = K-feldspar; grayish & whitish = quartz; blackish = amphibole.

The sample above doesn't show the foliation necessary to call a rock “gneiss”.  At the outcrop scale, foliation is present, and larger samples show hints of mineral segregation into bands (see photo below).


Compeau Creek Gneiss sample (~11 cm across) of Archean age from Co. Rd. 553 roadcut.  Pinkish-orangish = K-feldspar; quartz = gray; black = amphibole.



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