Ashe Metamorphic Suite

The Ashe Metamorphic Suite, also referred to as the Ashe Formation, was named after its type locality, Ashe County, North Carolina.[1] The Ashe Metamorphic Suite is located in the Eastern Blue Ridge providence that extends from North Carolina up to South-Western Virginia. It is a collection of metamorphic rocks of both sedimentary and volcanic origin. Zircon dating indicates an age of 470 to 335 Ma for the unit.[2] The protolith of the Ashe Metamorphic Suite was deposited during the Late Proterozoic and reaching its cooling age during the end of the Devonian. The Ashe Metamorphic Suite is overwhelmingly composed of amphibolites and mica schists.[3]

Stratigraphic range: 470-335 Ma
Outcrop of the Ashe Metamorphic Suite taken at Elk Knob State Park.
TypeGeological formation
AreaAppalachian Mountains
Location
LocationElk Knob State Park, North Carolina
Coordinates36°19′38″N 81°40′37″W
Type section
CountryUnited States

Nomenclature and Equivalent Units

[4] The Ashe Metamorphic Suite is thought to be correlative with a lower part of the Lynchburg Group in Virginia, the Mount Roger Formation in North Carolina and Virginia, and the Tallulah Falls Formation in Georgia.[5]

Geologic overview

The Ashe Metamorphic Suite is composed of a variety of different rocks such as metagreywackes, muscovite schist, quartzite,[3] graphitic schist, and mafic rocks such as amphibolite and hornblende gneiss.[5] The unit is overwhelmingly composed of mica schist with the southwest having intrusions of the ultramafic rocks.[4] The Ashe Metamorphic Suite overlies the Chilhowee Group of North Carolina and Tennessee or the mesoproterozoic Blue Ridge basement complex.[2] The Alligator Back Metamorphic Suite structurally overlies the Ashe Metamorphic Suite.[5] The Ashe Metamorphic Suite is intersected by the Grandfather Mountain window leaving a northeastern and southwestern region of the unit.

Petrology and Geochemistry
Ashe Metamoprhic Suite from Meat Camp, North Carolina.
Ashe Metamorphic Suite amphibolite from Meat Camp, NC.

Chemical composition

A whole rock analyses of 8 samples of Ashe Metamorphic Suite amphibolites demonstrates a basaltic origin of the amhibolites suggesting a mid-ocean ridge basalt protolith.[5]

Oxide

Weight %

 As-20 As-24 As-49 As-56 As-82 As-12 As-27 As-66
SiO2 51.69 50.67 49.31 49.85 51.00 47.89 50.79 50.93
TiO2 0.15 0.44 1.29 1.88 1.43 3.23 3.16 13.78
Al2O3 12.28 12.07 16.20 13.93 14.14 12.88 12.01 13.78
Fe2O3 12.89 14.88 10.66 14.15 12.15 16.44 16.06 15.03
MnO 0.15 0.18 0.16 0.20 0.17 0.21 0.22 0.20
MgO 8.83 7.95 7.35 6.68 7.22 12.78 5.54 4.31
CaO 11.64 11.13 9.38 10.03 10.32 9.79 9.24 6.76
Na2O 1.28 1.51 3.61 2.87 2.78 2.92 2.34 2.25
K2O 0.19 0.17 0.25 0.31 0.29 0.27 0.83 2.39
P2O5 0.02 0.06 0.17 0.33 0.27 0.44 0.43 0.84
Total 99.12 99.06 98.38 100.23 99.77 100.20 100.62 98.89
PPL Pelite
PPL Pelite
XPL Pelite
XPL Pelite
Thin section of a pelite from the Ashe Metamorphic Suite in plain polarized light and crossed polarized light.
PPL Amphibolite
PPL Amphibolite
XPL Amphibolite
XPL Amphibolite
Thin section of a amphibolite from the Ashe Metamorphic Suite in plain polarized light and crossed polarized light.

Mafic Rocks

The mafic rocks of the Ashe Metamorphic Suite are composed primarily of hornblende schists and gneiss. The main minerals composing these rocks are hornblende, quartz and plagioclase with minor garnet, biotite, epidote-zoisite and magnetite being present.[2]

Pelitic schists

The pelitic rocks of the Ashe Metamorohic Suite to the north of the Grandfather Mountain Window are primarily muscovite schists.[2] Depending on the metamorphic grade, the pelitic rocks can be consist of quartz, plagioclase, biotite, garnet, chlorite, staurolite, and kyanite.

Formation and origin

The Ashe Metamorphic Suite is confined between the opening of the Iapetus sea at the beginning of the Cambrian, and the closing of the ocean by in the Early Ordovician.[6] It was initially suggested the Ashe Metamorphic Suite was deposited on a rifted continental margin with a nonconformable contact with the underlying Cranberry Gneiss.[7] More recently the Ashe Metamorphic Suite is thought to be a subduction-related mélange with the contact between the Cranberry Gneiss and the Ashe Metamorphic Suite being a fault, or that the Ashe Metamorphic Suite was deposited as a back-arc basin.[8][9][5]

References
  1. "Geolex — Ashe publications". ngmdb.usgs.gov. Retrieved 2021-03-18.
  2. Abbott, R. N.; Greenwood, J. P. (July 2001). "Retrograde metamorphism of eclogite in the southern Appalachian Mountains, U.S.A.-A case involving seamount subduction?: Retrograde eclogite in the southern Appalachians". Journal of Metamorphic Geology. 19 (4): 433–443. doi:10.1046/j.0263-4929.2001.00321.x. S2CID 131406598.
  3. Bailey, Christopher M.; Coiner, Lorrie V. (2014-03-26). Elevating Geoscience in the Southeastern United States: New Ideas about Old Terranes: Field Guides for the GSA Southeastern Section Meeting, Blacksburg, Virginia, 2014. Geological Society of America. ISBN 978-0-8137-0035-9.
  4. Raymond, Loren A. (Oct 2015). "Formal Revision of the Ashe and Alligator Back Formation Names". Southeastern Geology. 51: 135–143.
  5. Misra, Kula C.; Conte, Jonathan A. (1991-06-01). "Amphibolites of the Ashe and Alligator Back Formations, North Carolina: Samples of Late Proterozoic-early Paleozoic oceanic crust". GSA Bulletin. 103 (6): 737–750. Bibcode:1991GSAB..103..737M. doi:10.1130/0016-7606(1991)103<0737:AOTAAA>2.3.CO;2. ISSN 0016-7606.
  6. Rankin, Douglas W. (1993). "The volcanogenic Mount Rogers Formation and the overlying glaciogenic Konnarock Formation; two late Proterozoic units in southwestern Virginia". USGS Professional Publication. doi:10.3133/b2029.
  7. Skipper, Hannah; Ranson, William A. (2020). "Geology and Blue Ridge Escarpment Evolution Along Bearcamp, Bear, and Bearwallow Creeks In Southwestern North Carolina and NorthWestern South Carolina". Geological Society of America Abstracts with Programs. Geological Society of America. doi:10.1130/abs/2020se-345247. S2CID 218806678. {{cite journal}}: Cite journal requires |journal= (help)
  8. Abbott, R. N.; Raymond, L. A. (1984-05-01). "The Ashe Metamorphic Suite, Northwest North Carolina; metamorphism and observations on geologic history". American Journal of Science. 284 (4–5): 350–375. Bibcode:1984AmJS..284..350A. doi:10.2475/ajs.284.4-5.350. ISSN 0002-9599.
  9. Greenawald, David; Stewart, Kevin G.; Gaynor, Sean (2017). "Amphibolite Zircon U-PB Geochronology in the Ashe Metamorphic Suite, Eastern Blue Ridge, North Carolina". Geological Society of America Abstracts with Programs. Geological Society of America. doi:10.1130/abs/2017am-305430. {{cite journal}}: Cite journal requires |journal= (help)
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