Watersmeet dome, Gogebic Co., Michigan, USAi

Regional Level Types
Watersmeet dome	Dome
Gogebic Co.	County
Michigan	State
USA	Country

The Watersmeet dome is composed of an Archean core about 25 km long in an E-W direction and about 8 km wide in a N-S direction. The dome is considered to be a mantled gneiss dome. Early Proterozoic rocks in the core are infolded with the Archean. Deformation of the Archean and adjacent Early Proterozoic rocks occurred during the Penokean orogeny. The rocks in the core were metamorphosed to the amphibolite facies, with a 15 km-wide zone of epidote amphibolite facies surrounding the core, and this in turn passes outward into greenschist facies. The Early Proterozoic rocks to the north of the dome have been overturned toward the northwest.

The gray rock of the dome is a tonalitic augen gneiss about 3560 Ma old (zircon data). It is a medium-gray, medium- to coarse-grained, biotiterich, irregularly layered rock with plagioclase augen. The alternating layers are feldspar-quartz-rich and biotite-rich. A modal analysis showed 48% plagioclase, 30% quartz, 8% K-feldspar, and 13% biotite. A possible protolith was dacitic volcanic rock. In places, biotite leucogranite veins 2590 Ma old (Rb/Sr and U-Pb methods) cut the gneiss. Secondary whole-rock and mineral isochrons give ages of 1750 to 1800 Ma and are thought to approximate the timespan of the Penokean event in this area. Parts of the ancient gneiss terrane are separated from the granite-greenstone terrane to the north by the Great Lakes Tectonic Zone. Whereas the Archean gneisses of the area may have been folded three times, the cross-cutting Late Archean leucogranite shows only a single foliation, that of the Penokean event. The gneiss is comparable in age to those of the Minnesota River Valley, about 3500 Ma old.

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