Diabase (Early Jurassic)
Diabase - Fine-grained to aphanitic dikes; medium- to coarsegrained, subophitic discordant stock-like intrusions of dark-greenish-gray to black diabase; and plugs of dark gray, concordant to discordant sheetlike, medium- to coarse-grained, quartz-rich to albite-rich granophyre (map unit Jg). The chilled margins of diabase masses are aphanitic to very fine grained. Diabase is dense, hard, and sparsely fractured. It is composed mostly of plagioclase (An50-70), clinopyroxene (mostly augite) and magnetite +/- ilmenite. Accessory minerals include apatite, quartz, alkali feldspar, hornblende, titantite, and zirocon. Olivine is rare. Within about 200 m (655 ft) above and 150 m (490 ft) below the large diabase sheets, red mudstones are typically metamorphosed into indurated, bluish-gray hornfels commonly with clots or crystals of tourmaline or cordierite, whereas argillitic siltstone is metamorphosed into brittle, black, very fine grained hornfels, Sheetlike intrusions are as much as 360 to 400 m (1,180-1,310 ft) thick. Dikes range in thickness from 3 to 15 m (10-50 ft) and several kilometers (miles) long. Thickness of the stocklike bodies is unknown.
Diabase - Concordant to discordant, predominantly sheet-like intrusions of medium- to fine-grained diabase and dikes of fine-grained diabase; dark-greenish-gray to black; subophitic texture. Dense, hard, sparsely fractured rock composed mostly of plagioclase (An50-70), clinopyroxene (mostly augite), and magnetite-ilmenite. Orthopyroxene (En75-80) is locally abundant in the lower part of the sheets. Accessory minerals include apatite, quartz, alkali feldspar, hornblende, sphene, zircon, and rare olivine. Diabase in the map area was derived primarily from high-titanium, quartz-tholeiite magma. Sedimentary rocks within about 300 m (984 ft) above and 200 m (656 ft) below major diabase sheets are thermally metamorphosed. Red mudstone is typically altered to indurated, bluish-gray hornfels with clots or crystals of tourmaline or cordierite. Gray argillitic siltstone is typically altered to brittle, black, very fine grained hornfels. Sills are 365 to 400 m (1,197-1,312 ft) thick. Dikes range in thickness from 3 to 10 m (10-33 ft) and are many kilometers long.
Diabase dikes (Late Proterozoic) (Late Proterozoic)
Diabase dikes (Late Proterozoic) (Ratcliffe, 1987; Puffer and others, 1991) - Medium- to dark greenish-gray, aphanitic to fine-grained dikes that are dense and hard. Contacts are typically chilled and sharp against enclosing country rock. Tholeiitic to alkalic composition; hypersthene a normative mineral. Composed principally of plagioclase (labradorite to andesine), augite, and ilmenite and (or) magnetite. Locallly occurring pyrite blebs are ubiquitous. Dikes range from a few cm to 20 m (an inch to 65 ft. wide) and as much as 5 km (3 mi) long. Occur throughout the Highlands, but intrude only Middle Proterozoic rocks.