Studies of the Lake Superior Pre-Cambrian by Accessory-Mineral Methods
Several hundred samples of pre-Cambrian rocks from the States bordering Lake Superior have been tested for heavy minerals and their ratios. Such ratios are believed to be of little value for correlation of masses of the same age or for distinction of those of different ages. Early pre-Huronian rocks on the south shore of Lake Superior and pre-Animikie intrusives north of the lake commonly have the “hyacinth” variety of accessory zircon, resembling the “purple” zircon described by Mackie from the pre-Cambrian of Scotland. Late pre-Huronian and Huronian rocks south of Lake Superior carry malacon alone, or more rarely malacon with some hyacinth that may be derived by incorporation of the older rocks. Intrusive rocks north of Lake Superior have not as yet yielded any abundant accessory malacon, and intrusives of the age of those bearing malacon in the southern areas may be rare north of the lake. Keweenawan igneous rocks on both sides of Lake Superior carry zircons that are characteristically euhedral simple crystals, colorless to pale yellow brown. These Keweenawan igneous rocks may, like the Huronian, have at places incorporated older rocks, but very few heavy separates have shown such contamination. The differences in those three types of zircons suggest very important correlations of certain granite masses, the age of which was not clearly settled by field relations. The dates of exposure of these igneous rocks are reflected in the sediments, showing the varieties of zircons derived from them. The detrital heavy mineral suites in all the Huronian and pre-Animikie sediments studied are so similar in the characters of the zircons and so irregular in the authigenic accessory minerals that individual Huronian formations cannot be distinguished by heavy mineral methods. Such methods, however, distinguish Huronian and older sediments from those of Keweenawan age and correlate certain formations of the Upper Keweenawan where the field observations left some uncertainty. In this respect the Keweenawan sediments resemble the Paleozoic and later sediments which have at places been correlated by heavy mineral work. Several generalizations as to igneous accessory minerals can be based on the data, especially those on the Keweenawan rocks: (1) The average relative percentages of accessory minerals is different for rocks of different composition even if the rocks are of the same age and appear to be genetically related; (2) The type of dominant zircon in Keweenawan rocks is remarkably constant regardless of rock composition—so long as zircon can be concentrated—and almost regardless of the texture and cooling history of the rock. The changes in heavy accessory minerals in weathering and sedimentation are striking, and the data in this region do not wholly agree with previous remarks as to which minerals are most rapidly attacked and which least by such processes. More work is needed.