2009-09-25_PERMIT FILE - M2009076 (31)

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• The quartz gneiss is light colored, fine to medium grained, and typically has a glassy luster. The gneissic structure of the rock results from slight differences in grain size in the laminae parallel to the rock layers. The rock contains as much as 80 percent quartz and some feldspar. Dark minerals, such as biotite and magnetite, are either sparse or absent. . AMPHIBOLITE AND ASSOCIATED CALC-SILICATE GNEISS Amphibolite commonly is exposed along the contact between the microcline gneiss and the biotite gneiss in the form of lenses locally more than 100 feet thick. Amphibolite also forms smaller layers and lenses in the microcline gneiss and, less commonly, in biotite gneiss and granite gneiss. The amphibolite is a dark-gray to black, fine- to medium-grained rock that contains hornblende and andesine in various proportions and small amounts of quartz. Biotite and pyroxene are common, though rarely are both present in the same specimen. Some varieties of amphibolite are massive and nearly structureless, others are gneissic and laminated. The gneissic structure is produced by alternating homblende- and plagioclase-rich layers and by the planar orientation of hornblende. Calc-silicate gneiss locally forms irregular masses or crosscutting veinlike structures in amphibolite. The contacts are ragged and the calc-silicate gneiss appears to be an alteration product of the amphibolite. The calc-silicate gneiss is mottled, light to dark colored, and fine to coarse grained. It contains calcium-rich garnet, quartz, abundant epidote, and some hornblende, plagioclase, and, locally, clinopyroxene. CALC-SILICATE GNEISS Calc-silicate gneiss, apparently unrelated to amphibolite, forms large bodies on the north and east sides • of Pewabic Mountain. The rock is mottled dark to light, crudely banded, and is fine to coarse grained. It is composed largely of diopside, epidote, quartz, scapolite, oligoclase, microcline, and arfvedsonite ( a sodium- rich amphibole) in various proportions. ORIGIN OF THE GNEISSIC ROCKS The gneissic rocks, with the exception of the granite gneiss, probably represent a thick succession of sedimentary rocks that were metamorphosed at high temperatures and pressures nearly equivalent to conditions of the upper range of the almandine-amphibolite facies, as defined by Fyfe, Turner, and Verhoogen (1958, p. 230-232). The biotite gneiss probably represents metamorphosed shale and interbedded sandstone, for this unit is marked by alternating sillimanitic and nonsillimanitic layers. The layers have the appearance of beds, and the sillimanite probably reflects the high aluminum content that characterizes most shales. The origin of the microcline gneiss is more debatable. Units of microcline gneiss are conformable with units of biotite gneiss and contain abundant conformable layers and lenses of amphibolite as well as some layers of biotite gneiss. These features are most easily explained as having resulted from sedimentary processes. Conceivably, the microcline gneiss represents metamorphosed arkose. Amphibolite is most abundantly exposed along the contacts between the microcline gneiss and biotite gneiss. This indicates that it was once a sedimentary rock. The composition of the amphibolite suggests formation from impure dolomitic sedimentary rocks; it could also represent metamorphosed basalt. One variety of calc-silicate gneiss probably formed by recrystallization of amphibolite, but the other varieties may represent calcareous sedimentary layers in the predominantly noncalcareous shales and sands that formed the biotite gneiss. The quartz gneiss layers probably represent quartz-rich sandstone beds. Granite gneiss increases in abundance southwestward across the areas seemingly largely at the expense of biotite gneiss. This would indicate that the granite gneiss formed largely by replacement of the biotite gneisses; the ragged contacts observed in the outcrops support this interpretation. However, the possibilities that the observed regional change in composition represents a sedimentary facies change or that the rock was injected from a magmatic source are not precluded. • 10