The Ashe Metamorphic Suite, also referred to as the Ashe Formation, was named after its type locality,

Ashe County Ashe County ( ) is a county located in the U.S. state of North Carolina. The population was 26,577 at the 2020 census. Its county seat is Jefferson. History Historical evidence shows that Ashe County was inhabited by Native Americans, wh ...

North Carolina North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...

. The Ashe Metamorphic Suite is located in the Eastern Blue Ridge providence that extends from North Carolina up to South-Western

Virginia Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Southeastern United States, Southeastern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States between the East Coast of the United States ...

. It is a collection of metamorphic rocks of both sedimentary and volcanic origin. Zircon

dating Dating is a stage of Romance (love), romantic relationships in which individuals engage in activity together, often with the intention of evaluating each other's suitability as a partner in a future intimate relationship. It falls into the cate ...

indicates an age of 470 to 335 Ma for the unit. The

protolith A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and ...

of the Ashe Metamorphic Suite was deposited during the

Late Proterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era and succeeded b ...

and reaching its cooling age during the end of the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

. The Ashe Metamorphic Suite is overwhelmingly composed of amphibolites and mica schists.

Nomenclature and Equivalent Units

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 Georgia most commonly refers to: * Georgia (country), a country in the South Caucasus * Georgia (U.S. state), a state in the southeastern United States Georgia may also refer to: People and fictional characters * Georgia (name), a list of pe ...

Geologic overview

The Ashe Metamorphic Suite is composed of a variety of different rocks such as metagreywackes,

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage y ...

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

quartzite Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...

, graphitic schist, and mafic rocks such as

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose ...

and

hornblende Hornblende is a complex silicate minerals#Inosilicates, inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common ...

gneiss Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under p ...

. The unit is overwhelmingly composed of

mica schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

with the southwest having intrusions of the ultramafic rocks. The Ashe Metamorphic Suite overlies the Chilhowee Group of North Carolina and Tennessee or the mesoproterozoic Blue Ridge basement complex. The Alligator Back Metamorphic Suite structurally overlies the Ashe Metamorphic Suite. The Ashe Metamorphic Suite is intersected by the Grandfather Mountain window leaving a northeastern and southwestern region of the unit.

Petrology and Geochemistry

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

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.

Pelitic schists

The pelitic rocks of the Ashe Metamorohic Suite to the north of the Grandfather Mountain Window are primarily muscovite schists. 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 The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

, and the closing of the ocean by in the Early

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

. It was initially suggested the Ashe Metamorphic Suite was deposited on a rifted continental margin with a nonconformable contact with the underlying Cranberry Gneiss. 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 A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...

.{{Cite journal, last1=Greenawald, first1=David, last2=Stewart, first2=Kevin G., last3=Gaynor, first3=Sean, date=2017, title=Amphibolite Zircon U-PB Geochronology in the Ashe Metamorphic Suite, Eastern Blue Ridge, North Carolina, url=http://dx.doi.org/10.1130/abs/2017am-305430, journal=, series=Geological Society of America Abstracts with Programs, publisher=Geological Society of America, doi=10.1130/abs/2017am-305430, url-access=subscription

References

Geologic formations of North Carolina Geologic formations of Virginia