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

68 KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS, SOUTHEASTERN UNITED STATES
to the northwest, away from the axis of the much
larger South Fork anticline. The Yorkville quartz
monzonite has been intruded into the nearly vertical
east limb of this anticline.
On the basis of these major folds two principal
groups of metamorphic rocks are established: an
older group including biotite schist and gneiss,
hornblende gneiss, metagabbro, and metatonalite,
and a younger group including white mica schist,
schistose pyroclastic rock, schistose conglomerate,
chloritoid schist, manganiferous schist, and highalumina
quartzite (grading from sillimanite quartzite
to kyanite quartzite depending on the rank of
metamorphism).
Biotite schist and gneiss, derived from volcanic,
sedimentary, and igneous rocks are the most abundant,
and probably, in large part, the oldest rocks.
They occur at the core of the South Fork anticline,
in the vicnity of Gasset Lake, and in two narrow
areas along the limbs of the northern end of
the Sherrars Gap syncline. Hornblende gneiss, derived
largely from volcanic rocks, occurs principally
in the core of the South Fork anticline where it
is interlayered with biotite schist. Two metamorphosed
intrusive rocks occur in the southeastern
part of the Crowders Mountain-Henry Knob area;
tonalite (metatonalite), metamorphosed in part to
biotite gneiss, locally intrudes biotite schist; quartz
gabbro (metagabbro), largely metamorphosed to
hornblende gneiss, intrudes tonalite. This entire
group of older rocks may represent one episode of
related sedimentation, volcanism, and intrusion.
The younger group of rocks is exposed in the
trough and along the limbs of the Sherrars Gap
syncline, in the vicinity of Crowders Mountain, and
along the east limb of the South Fork anticline.
These younger rocks may be separated from the
older group by an unconformity because the marker
beds of kyanite and sillimanite quartzites and associated
white mica schist on the east limb of the
South Fork anticline lie on biotite gneiss, metatonalite,
and metagabbro, whereas on the west limb
the younger group lies entirely on biotite gneiss.
Plate 7 does not, in many places, present a consistent
stratigraphic and structural picture of either
group of rocks. Some of the schistose pyroclastic
rock in the northwestern part of the area may have
been mapped as chloritoid schist because pyroclastic
texture, the main distinguishing feature between
the two rocks, was obliterated by metamorphism or
weathering. White mica schist and some biotite
schist along the east limb of the South Fork anticline
may represent metamorphosed schistose pyroclastic
rock or chloritoid schist. Only the least
metamorphosed tonalite was mapped as metatonalite;
some white mica schist and much biotite
gneiss may represent highly sheared and metamorphosed
tonalite. The lenticularity of the quartzite
beds makes their correlation difficult. Their
lenticularity may be in part an original sedimentary
feature but may also be in part the result of flowage
and rupture during metamorphism.
Two structural interpretations of the geology of
the Crowders Mountain-Henry Knob area are given
to explain the distribution of kyanite quartzite, sillimanite
quartzite, and manganiferous schist within
the framework of the two major folds. According
to interpretation 1 there is 1 manganiferous schist
bed and 1 high-alumina quartzite bed; interpretation
2 proposes the presence of 1 manganiferous
schist bed and 3 beds of high-alumina quartzite.
Interpretation 1. Under interpretation 1 all the
outcrops of high-alumina quartzite and barren
quartzite would result from repetition of a single
quartzite bed by folding and faulting. In the western
part of the area the high-alumina quartzite
"bed" actually consists of 2 parallel layers of kyanite
quartzite and kyanite conglomerate separated
by 20 to 100 feet of schist; at Crowders Mountain
the bed is highly folded and thickened, and includes
much nonkyanitic quartzite; on the east limb of the
South Fork anticline the bed is apparently repeated
by an isoclinal syncline, for it occurs as thin parallel
lenses of sillimanite and kyanite quartzites.
The suggestion by Keith and Sterrett (1931) that
the manganiferous schist is older than the kyanite
quartzite is followed here, because the age relationships
of these units could not be determined within
the area shown on plate 7. Manganiferous schist
seems to lie below kyanite quartzite at the southwest
end of the Sherrars Gap syncline (fig. 42). However,
the distribution of manganiferous schist around
the high-alumina quartzite of the South Fork anticline
suggests that manganiferous schist is the
younger unit. Except for a small lens of manganiferous
schist at the north end of Crowders Mountain,
this rock is absent from the highly folded
areas proposed by interpretation 1. This absence
may reflect nondeposition, the shearing out of
manganiferous schist along the limbs of very tight
and complex folds, or the manganiferous schist may
be present in thin lenses but covered by kyanite
quartzite talus.
Schistose pyroclastic rock and chloritoid schist occur
at two stratigraphic positions, according to
both interpretations: an upper sequence in the