Item talk:Q148319

Geology of part of the southern complex, Marquette district, Michigan

The southern complex, south of the Marquette synclinorium in the Marquette district of Michigan, is dominantly granitic.  The granitic parts of the complex have Rb-Sr ages of about 2.5 b.y. and are classed as of Precambrian W age. The rocks are divided into two major units: (1.) Bell Creek Gneiss consisting mostly of coarse megacrystic granitic rocks and lesser amounts of mafic gneiss and metasedimentary rocks in layers concordant to the foliation of the granitic gneiss, and (2) Compeau Creek Gneiss, previously described to the east, which is mostly medium-grained granitic gneiss and lesser amounts of massive granitic rocks and locally abundant xenoliths of mafic and metasedimentary rocks. Compeau Creek Gneiss intrudes Bell Creek Gneiss throughout the area. Mafic and metasedimentary rocks in the Bell Creek Gneiss are interpreted as remnants of a greenstone belt. The granitic parts of the Bell Creek Gneiss are of problematic origin. Their
composition suggests a magmatic origin, but they do not intrude any rocks with which they are in contact; they form concordant layers in all
observed places. Granitic parts of the Compeau Creek Gneiss are suspected to be highly altered and partly melted sedimentary rocks.
The mafic parts are probably xenoliths of mafic rocks from the Bell Creek Gneiss.