Chromite in Lower Pennsylvanian Nuttall Sandstone from West
Virginia--Possible Blue
Ridge Province Source
J. T. O'Connor
Detrital chromite grains from heavy mineral separates of three samples of the
Lower Pennsylvanian Nuttall Sandstone Member of the New River Formation were
examined by use of a reflected-light petrographic microscope, a scanning
electron microscope, and an electron microprobe. Some textures observed
microscopically in the chromite grains are similar to those in chromite grains
in dunite from the Blue
Ridge province of North Carolina that has undergone
regional metamorphism.
Additional exsolution features are also observed in the chromites of this
study. The major-oxide compositions of the detrital chromite grains vary more
widely than those of the chromites of the Blue
Ridge, possibly due to a larger
sampling base. The detrital grain compositions [Cr/(Cr + Al), Mg/(Mg + Fe+2)]
plot near the fields of the
Blue
Ridge chromites, although the appearance of
both more aluminum-rich and magnesium-poor compositions possibly indicate a
higher facies of metamorphism than the
Blue
Ridge samples studied to date.
Paleocurrent directions in the Nuttall Sandstone Member in the area of sample
collection indicate that the source area for the sandstone could include part of
the Blue
Ridge province. Trace-mineral identification in the heavy mineral suite
of the sandstone includes some of the accessory minerals (tourmaline, zircon,
kyanite, garnet) of the rocks surrounding the chromite-bearing
Blue
Ridge
ultramafic bodies. The writer suggests that the source of the Nuttall Sandstone
Member included, among others, rocks equivalent to the dunites from the
Blue
Ridge province. The absence of feldspar and other major components of the
surrounding
Blue
Ridge host rocks in the Nuttall Sandstone Member implies that
the weathering regime at the time of deposition, leaving only the most resistant
detritus to e supplied to the Appalachian basin in original form, was very
pervasive.
AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.