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

80 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES
mined through the Larsen method by H. W. Jaffee
(oral communication, 1956) as 260 million years,
and because the quartz monzonite shows no signs
of having- been metamorphosed or altered by hypogene
processes and therefore was emplaced during
or after the last orogeny in this part of the
Piedmont.
Numerous long thin septa of metamorphic rocks
occur in the Yorkville quartz monzonite (pi. 7; fig.
42). These septa represent remnants of the metamorphic
sequence on the east limb of the South
Fork anticline (fig. 42). The principal metamorphic
rocks in these various septa are biotite-sillimanite
schist, biotite gneiss, sillimanite quartzite, garnetquartz-biotite
schist, manganiferous schist and finegrained
spessartite rock, pyroxene granulite, hornblende
gneiss, and corundum gneiss. Minor
amounts of anthophyllite gneiss and cordierite
gneiss were found as float. The corundum gneiss is
particularly interesting because corundum crystals
are generally imbedded in muscovite or in microcline
with a sheath of muscovite about the corundum
(fig. 50). Corundum may have formed from
FIGUKE 50. Photomicrograph of corundum gneiss from septum 1.3 miles
northwest of Pisgah Church, Gaston County N.C. The large euhedral crystal
of corundum (c) is partially replaced by a sheath of muscovite (m). Other
minerals are microcline (mi) and magnetite (mt). Crossed nicols.
aluminum released when muscovite was transformed
to microcline. The sheath of muscovite surrounding
the corundum is probably a still later reaction
between corundum, microcline, and water.
DIABASE
Steeply dipping dikes of fine- to medium-grained
diabase cut diagonally across the regional grain of
the rocks in the Crowders Mountain-Henry Knob
area (pi. 7). These dikes range from a few thousand
feet to 4 miles in length and from a few
inches to about 60 feet in width. They cut the metamorphic
rocks at places where kyanite quartzite
beds are very thin or absent. Diabase dikes cut the
Yorkville quartz monzonite and have been intruded
along the contact between the quartz monzonite
and the metamorphic rocks. The composition and
the texture of the diabase are uniform: the principal
constituents are labradorite, augite, and olivine;
ophitic texture is common. A Triassic age is assumed
for the emplacement of the diabase because
the dikes show no evidence of deformation and because
they are petrographically identical to diabase
dikes of known Triassic age.
STRUCTURE
Two major folds in the Crowders Mountain-
Henry Knob area, the South Fork anticline and the
Sherrars Gap syncline (pi. 7; fig. 42), are delineated
by the distribution of the outcrops of highalumina
quartzite and manganiferous schist. The
meager evidence for the synclinal and anticlinal
nature of these folds has been summarized in the
section on stratigraphy. In their southern parts
the axes of both these folds trend N. 40° E. In
the northern part the axis of the South Fork anticline
turns northward and trends parallel to the
contact of the Yorkville quartz monzonite. The anticline
is asymmetrical the east limb being nearly
vertical and the west limb (east limb of the Sherrars
Gap syncline) dipping steeply northwest. The
South Fork anticline plunges north at probably a
rather high angle. The Yorkville was emplaced
into the east limb of the South Fork anticline with
the result that many of the beds on the east limb
persist as septa in the quartz monzonite.
The Sherrars Gap syncline, which plunges
northeast in its southwestern half, has a sinuous
trend. In the southern part the axis of this fold
trends N. 40° E., in the middle, N. 60° E., and in
the northern part, N. 10° E. The southern part of
the syncline is overturned to the southeast with the
axial plane dipping steeply northwest; in the cen-