COMPEAU CREEK GNEISS
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.