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• CALL-SILICATE GNEISS <br />Calc-silicate gneiss, apparently unrelated to amphibolite, forms large bodies on the north and east sides <br />of Pewabic Mountain. The rock is mottled dark to light, crudely banded, and is fine to coarse grained. It is <br />composed largely of diopside, epidote, quartz, scapolite, oligoclase, microcline, and arfvedsonite ( a sodium- <br />rich amphibole) in various proportions. <br />ORIGIN OF THE GNEISSIC ROCKS <br />The gneissic rocks, with the exception of the granite gneiss, probably represent a thick succession of <br />sedimentary rocks that were metamorphosed at high temperatures and pressures nearly equivalent to conditions <br />of the upper range of the almandine-amphibolite facies, as defined by Fyfe, Turner, and Verhoogen (1958, p. <br />230-232). The biotite gneiss probably represents metamorphosed shale and interbedded sandstone, for this unit <br />is marked by alternating sillimanitic and nonsillimanitic layers. The layers have the appearance of beds, and the <br />sillimanite probably reflects the high aluminum content that characterizes most shales. The origin of the <br />microcline gneiss is more debatable. Units of microcline gneiss are conformable with units of biotite gneiss and <br />contain abundant conformable layers and lenses of amphibolite as well as some layers of biotite gneiss. These <br />features are most easily explained as having resulted from sedimentary processes. Conceivably, the microcline <br />gneiss represents metamorphosed arkose. Amphibolite is most abundantly exposed along the contacts between <br />the microcline gneiss and biotite gneiss. This indicates that it was once a sedimentary rock. The composition of <br />the amphibolite suggests formation from impure dolomitic sedimentary rocks; it could also represent <br />metamorphosed basalt. One variety of calc-silicate gneiss probably formed by recrystallization of amphibolite, <br />but the other varieties may represent calcareous sedimentary layers in the predominantly noncalcareous shales <br />and sands that formed the biotite gneiss. The quartz gneiss layers probably represent quartz-rich sandstone beds. <br />• Granite gneiss increases in abundance southwestward across the areas seemingly largely at the expense <br />of biotite gneiss. This would indicate that the granite gneiss formed largely by replacement of the biotite <br />gneisses; the ragged contacts observed in the outcrops support this interpretation. However, the possibilities that <br />the observed regional change in composition represents a sedimentary facies change or that the rock was <br />injected from a magmatic source are not precluded. <br />GRANITIC ROCKS <br />Granitic rock units are intrusive igneous rocks and were emplaced in the following order: <br />Granodiorite, quartz diorite, and biotite-muscovite granite. The granodiorite and quartz diorite occur as small, <br />nearly conformable bodies, and the biotite-muscovite granite forms small sills, phacoliths, and a few thin <br />crossdressing dikes. <br />GRANODIORITE <br />Several small bodies of granodiorite crop out in the southeastern part of the district. The granodiorite <br />exposed here is fine grained and schistose and is similar to the granodiorite found near the borders of larger <br />bodies in the Chicago Creek area (Harrison and Wells, 1959, p. 12). The rock is dark gray and composed of <br />about 15 percent quartz, 40 percent oligociase-andesine, 15 percent microcline, 20 percent biotite, and 10 <br />percent accessory minerals-sphene, magnetite, apatite, allanite, and epidote. <br />QUARTZ DIORITE <br />Quartz diorite forms several small nearly conformable bodies on the southeast side of the district. The <br />rock is similar in geologic occurrence and texture to the granodiorite but is darker in color. The central parts of <br />the larger bodies of quartz diorite are dark gray, medium grained, and equigranular, whereas the margins are <br />typically well foliated. The massive rocks contain as much as 70 percent combined homblende, clinopyroxene, <br />0 and biotite, as much as 40 percent plagioclase (andesine), and generally less than 15 percent quartz. Accessory