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 58 02 North, 82 17 01 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).




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