Watersmeet dome, Gogebic Co., Michigan, USA
|Latitude & Longitude (WGS84):||46° 19' 30'' North , 89° 11' 10'' West|
|Latitude & Longitude (decimal):||46.32500,-89.18611|
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.
6 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
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|Neoarchean - Paleoarchean|
2500 - 3600 Ma
|Migmatitic gneiss and amphibolite (Late to Early Archean)|
Age: Archean (2500 - 3600 Ma)
Description: Varied gneisses of mostly unknown age in cores of gneiss domes and fault-bounded uplifts (Archean gneiss terranes). Except for the Watersmeet dome (Late to Early Archean), all dated rocks are Late Archean. Includes granite of Late Archean age that transgresses gneisses and amphibolite.
Comments: continental margin assemblage (north of Niagara Fault)
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052.