DAY BOOK DUNITE
In orogenic belts, olivine-bearing rocks often have
the olivine component altered and metamorphosed, usually into serpentine.
The Day Book Dunite is an exception to this. It is located in the
Blue Ridge Province of the southern Appalachian Mountains in North Carolina,
USA.
Dunites are >90% olivine varieties of peridotites,
which are ultramafic intrusive igneous rocks. The Day Book Dunite of
western North Carolina appears to represent a metamorphosed sliver of mantle
rocks, but it has not been serpentinized. In the rocks shown below, the
greenish material is forsterite olivine (magnesium-rich (Mg,Fe)2SiO4).
The black material is chromite (iron chromium oxide - FeCr2O4).
Minor chlorite and magnesite are also present.
These rocks have been metamorphosed somewhat (and thus
should really be called metadunites, not dunites). The chlorite is
a metamorphic mineral, and the smaller olivine crystals in this material have
been attributed to metamorphic recrystallization. Metamorphism of the Day
Book Dunite took place during the Ordovician (?).
Chromitic metadunite (recrystallized) (8.0 cm across) from the Day Book
Dunite. Locality: Green Mountain Mine, ~2 km southeast of the town
of Day Book, northern Yancey County, Spruce Pine District, Blue Ridge, southern
Appalachians, western North Carolina, USA (35¡ 58Õ 02Ó North, 82¡ 17Õ 01Ó
West).
Metadunite (green) with band of chromitite (black) from the Day Book Dunite.
Specimen owned by the geology department of Ohio State
University (Columbus campus).