Kyanite, Sillimanite, and Andalusite Deposits of the Southeastern ...

KYANITE DEPOSITS 43
tain (fig. 29). The thin isolated layers of quartzite
and kyanite quartzite at the southern end of the
Whispering Creek anticline may be a variety of
larger scale boudinage structure.
METAMORPHISM
Except for the graphitic schist south of Dillwyn,
the pre-Triassic rocks in the mapped area have all
been regionally metamorphosed to about the same
degree. The various mineral assemblages were
formed in rocks of different chemical composition
under conditions of the amphibolite facies; they do
not represent mineral zoning caused by important
differences in the intensity of metamorphism. Characteristic
minerals are kyanite, quartz, biotite,
muscovite, garnet, hornblende, epidote, and plagioclase.
Less abundant minerals are staurolite, actinolite,
tremolite, anthophyllite, talc(?), and chlorite;
small amounts of sillimanite are present in a few
places, and scapolite was found at two localities.
All these minerals, except possibly scapolite, seem
to have been formed primarily from the original
constituents of the country rock; however, some
5 Feet
FIGURE 29. Vertical sections showing folds and boudinage structure in
kyanite quartzite in opencut mine of Kyanite Mining Corp., Baker Mountain,
Prince Edward County, Va.
transfer of material took place during metamorphism.
Microcline porphyroblasts occur in biotite
gneiss in some parts of the region, especially in the
Baker Mountain-Madisonville area; microcline may
have formed by introduced potassium or from potassium
that was mobilized during metamorphism.
The Ordovician(?) rocks that may be in an extension
of the Arvonia syncline (the graphitic
schist south of Dillwyn and quartz-muscovite schist
northeast of Dillwyn) are fine grained and appear
less highly metamorphosed than the gneisses in the
mapped area to the east. Small crystals of garnet
and kyanite are present, however, in some of the
quartz-muscovite schist near Dillwyn. Fossiliferous
slates are exposed in quarries in the vicinity of
Arvonia, 12 miles northeast of Dillwyn.
It seems likely that the peak of regional metamorphism
was reached early in the Paleozoic era,
before or during deposition of the Ordovician rocks.
The Arvonia slate may have been folded and metamorphosed
in the declining stages of orogeny.
Some of the metamorphic minerals have undergone
minor alteration to minerals that are stable at
lower temperatures. The most common alteration
of this type is the slight alteration of biotite to
chlorite, which is found throughout the region.
Hornblende is also partly altered to chlorite but to a
lesser extent than biotite. Kyanite is altered to
muscovite and clay minerals at a few places. This
minor retrogressive metamorphism probably took
place through the action of fluids in the declining
stages of metamorphism when the temperatures
had decreased. Jonas (1932) has advanced the view
that profound retrogressive metamorphism took
place during late Paleozoic diastrophism; however,
we have not found evidence of major retrogressive
metamorphism.
GEOLOGIC HISTORY
The geologic history of the region is incompletely
understood because time did not permit a study of
the belt of Ordovician rocks in the Arvonia syncline
just west of the north end of the kyanite district.
The relationships between the stratigraphy, structure,
and metamorphism of these two groups of
rocks is not altogether clear. Present interpretation
of the geologic history of the mapped and adjacent
areas is as follows:
Late Precambrian(?) and early Paleozoic(?): Deposition of a
thick series of graywacke, basalt, shale and (or) silicic
volcanic rocks, and extensive thin beds of clayey sandstone.
This probably took place during the period of deposition of
the Lynchburg gneiss and basalt flows of the Catoctin
formation to the west. Deformation and metamorphism,
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