Biotite gneiss

What the map says

Gray, medium-grained, equigranular, well-foliated gneiss typically containing approximately 25–50 percent quartz, 20–30 percent plagioclase (approximately An30), 0–30 percent microcline, 10–15 percent biotite, 0–15 percent muscovite, 0–10 percent sillimanite, 0–5 percent hornblende, 1–2 percent opaque minerals, and a trace zircon. Most outcrops contain 5–20 percent granitic layers (leucosomes). Sillimanite, where abundant, occurs in fibrous, elongate, light-colored aggregates or clots as wide as about 1 cm. Migmatitic biotite gneiss in Clear Creek gave preliminary U-Pb zircon date on nonmelt phase of 1,773±18 Ma; near Pine, just south of map area, similar rock gave U-Pb zircon date of 1,780±9 Ma (W.R. Premo, unpub. data, 2005)

1:100,000 sheet: Kellogg and others, 2008, USGS Geologic Investigations Series 3000 (31 crags). A contact line at that scale is about 100 m wide on the ground.

Mostly gray to dark-gray, fine- to medium-grained, moderately foliated to strongly foliated and layered gneiss and schist. Composed of quartz, plagioclase, microcline, and biotite; locally contains hornblende, sillimanite, and garnet. Accessory minerals are rutile, sphene, apatite, zircon, and magnetite. Sillimanite-bearing biotite schist (metapelite) crops out in and near the Twin Lakes mining district. Migmatitic in most places; contains numerous light-colored layers and lenses (leucosomes) typically 0.1–10 cm thick, although locally may be much thicker. In most places, leucosome bodies form less than half of rock, have sharp contacts with host rock, show much pinch and swell, and in some places are strongly folded. Leucosomes are composed of equigranular, massive to weakly foliated, white to light-gray microcline-plagioclase-quartz rock containing less than 5 percent biotite; accessory minerals are muscovite, opaque minerals, sphene, apatite, garnet, and zircon. Formation of leucosomes may be due to either injection or in situ partial melting (anatexis) (Johannes and Gupta, 1982; Olsen, 1982). In the case of partial melting, biotite gneiss adjacent to leucosomes commonly has biotite-rich selvages representing refractory material remaining after partial melting. Some areas mapped as biotite gneiss may include small bodies of granitic rock. Unit undated in map area, but similar rocks in the Gore Range and western Front Range, northeast of the map area, have a U-Pb zircon age of about 1,750–1,785 Ma (Kellogg and others, 2008, 2011), which represents the age of the source rocks from which detritus that comprises the sedimentary protolith was derived. In the same areas, U-Pb zircon geochronology indicates that formation of migmatite (partial melting) occurred about 1,695 Ma, which is also the approximate time at which many pegmatites in and near these areas formed (Kellogg and others, 2011)

1:100,000 sheet: Kellogg and others, 2017, U.S. Geological Survey Scientific Investigations Map 3382 (4 crags). A contact line at that scale is about 100 m wide on the ground.

Served by Macrostrat (CC BY 4.0). The wording is the map's own, quoted as a fact about the mapped unit, not about any one wall.