The Ashe Metamorphic Suite, Northwest North Carolina; metamorphism and observations on geologic history

Abbott, Richard N.; Raymond, Loren A.

doi:10.2475/ajs.284.4-5.350

Cited by 42 publications

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“…In the literature, they have been described as mélanges (e.g. Abbott and Raymond 1984) or Alpine-type ultramafic rocks, and they contrast markedly with the large, nearly complete Penrose-type ophiolite sections of the northern Appalachians that formed in a suprasubduction zone setting. They are pretectonic with respect to the pervasive Taconic deformation, and few primary textural and structural features have survived amphibolite to granulite-grade regional metamorphism, although compositional layering is locally preserved in some ultramafic rocks (Swanson et al 2005).…”

Section: Virginia -Carolinamentioning

confidence: 99%

The Ocean – Continent Transition Zones Along the Appalachian – Caledonian Margin of Laurentia: Examples of Large-Scale Hyperextension During the Opening of the Iapetus Ocean

Chew

Staal

2014

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A combination of deep seismic imaging and drilling has demonstrated that the ocean-continent transition (OCT) of present-day, magma-poor, rifted continental margins is a zone of hyperextension characterized by extreme thinning of the continental crust that exhumed the lowermost crust and/or serpentinized continental mantle onto the seafloor. The OCT on present-day margins is difficult to sample, and so much of our knowledge on the detailed nature of OCT sequences comes from obducted, magma-poor OCT ophiolites such as those preserved in the upper portions of the Alpine fold-and-thrust belt. Allochthonous, lens-shaped bodies of ultramafic rock are common in many other ancient orogenic belts, such as the Caledonian – Appalachian orogen, yet their origin and tectonic significance remains uncertain. We summarize the occurrences of potential ancient OCTs within this orogen, commencing with Laurentian margin sequences where an OCT has previously been inferred (the Dalradian Supergroup of Scotland and Ireland and the Birchy Complex of Newfoundland). We then speculate on the origin of isolated occurrences of Alpine-type peridotite within Laurentian margin sequences in Quebec – Vermont and Virginia – North Carolina, focusing on rift-related units of Late Neoproterozoic age (so as to eliminate a Taconic ophiolite origin). A combination of poor exposure and pervasive Taconic deformation means that origin and emplacement of many ultramafic bodies in the Appalachians will remain uncertain. Nevertheless, the common occurrence of OCT-like rocks along the whole length of the Appalachian – Caledonian margin of Laurentia suggests that the opening of the Iapetus Ocean may have been accompanied by hyperextension and the formation of magma-poor margins along many segments.SOMMAIREDes travaux d’imagerie sismique et des forages profonds ont montré que la transition océan-continent (OCT) de marges continentales de divergence pauvre en magma exposée de nos jours, correspond à une zone d’hyper-étirement tectonique caractérisée par un amincissement extrême de la croûte continentale, qui a exhumé sur le fond marin, jusqu’à la tranche la plus profonde de la croûte continentale, voire du manteau continental serpentinisé. Parce qu’on peut difficilement échantillonner l’OCT sur les marges actuelles, une grande partie de notre compréhension des détails de la nature de l’OCT provient d’ophiolites pauvres en magma d’une OCT obduite, comme celles préservées dans les portions supérieures de la bande plissée alpine. Des masses lenticulaires de roches ultramafiques allochtones sont communes dans de nombreuses autres bandes orogéniques anciennes, comme l’orogène Calédonienne-Appalaches, mais leur origine et signification tectonique reste incertaine. Nous présentons un sommaire des occurrences d’OCT potentielles anciennes de cet orogène, en commençant par des séquences de la marge laurentienne, où la présence d’OCT a déjà été déduites (le Supergroupe Dalradien d’Écosse et d'Irlande, et le complexe de Birchy de Terre-Neuve). Nous spéculons ensuite sur l'origine de cas isolés de péridotite de type alpin dans des séquences de marge des Laurentides du Québec-Vermont et de la Virginie-Caroline du Nord, en nous concentrant sur les unités de rift d'âge néoprotérozoïque tardif (pour éviter les ophiolites du Taconique). La conjonction d’affleurements de piètre qualité et de la déformation taconique omniprésente, signifie que l'origine et la mise en place de nombreuses masses ultramafiques dans les Appalaches demeureront incertaines. Néanmoins, la présence fréquente de roches de type OCT tout le long de la marge Calédonnienne-Appalaches de Laurentia suggère que l'ouverture de l'océan Iapetus peut avoir été accompagnée d’hyper-étirement et de la formation de marges pauvres en magma le long de nombreux segments.

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Section: Virginia -Carolinamentioning

confidence: 99%

The Ocean – Continent Transition Zones Along the Appalachian – Caledonian Margin of Laurentia: Examples of Large-Scale Hyperextension During the Opening of the Iapetus Ocean

Chew

Staal

2014

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“…Schists and gneisses preserve a polyphase metamorphic history (Fig. 1) in mineral assemblages and structures (Butler 1973(Butler , 1991Abbott and Raymond 1984;Adams et al 1995). Peak metamorphism was assumed to be middle to upper amphibolite facies (Butler 1973;Abbott and Raymond 1984), but detailed mapping in the Spruce Pine thrust sheet revealed an earlier eclogite-facies metamorphic assemblage locally preserved near the base of the thrust sheet (Adams et al 1995;Adams and Trupe 1997).…”

Section: Geological Backgroundmentioning

confidence: 99%

Mineralogy and petrogenesis of pegmatites in the Spruce Pine District, North Carolina, USA

Swanson¹,

Veal²

2012

J. Geosci.

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Granitic rocks (mainly granodiorites) of the Spruce Pine Plutonic Suite occur as small plutons and numerous pegmatite bodies intruded into amphibolite-grade schists and gneisses of the Ashe Formation. Intrusion ages range from 377 to 404 Ma and are near the peak of amphibolite-facies metamorphism in the country rocks. The granodiorites have a mortar texture with coarse (~ 1 cm) grains of feldspar, quartz and muscovite enclosed in a finer grained matrix of the same phases. Large (10-30 cm) crystals of perthitic K-feldspar and plagioclase occur in the granodiorite. Pegmatites occur within the granodiorite and as isolated intrusions within the country rocks. Contacts between pegmatites and host granodiorites range from sharp, cross-cutting to completely gradational. Some of the plutons contain up to 50 % pegmatite. Pegmatite mineralogy is similar to the granodiorites; major amounts of feldspars and quartz, accessory amounts of muscovite, garnet, and epidote. Secondary minerals, formed by subsolidus recrystallization, include epidote, muscovite, quartz, and grossular garnet. A few of the pegmatites are more fractionated and contain tourmaline and beryl. Compositions of feldspars, muscovite, and garnet show some regional variation, but are generally similar for pegmatites and associated granodiorite. About half of the pegmatites are unzoned. Zoning in the pegmatites is simple with a quartz core, an intermediate zone of graphic granite, plagioclase and muscovite and an outer finer grained zone that locally shows mineral layering parallel to the contact. The abundance of pegmatitic textures relative to granitoid textures in the Spruce Pine Plutonic Suite is related to the depth of crystallization. Spruce Pine granitoids crystallized at depths of 20-30 km (0.7 to 1.1 GPa ) based on feldspar thermobarometry and experimentally determined melting relations. Skeletal crystals of quartz and feldspar along with plumose muscovite clusters are common in the Spruce Pine pegmatites. Water content of magmas at such high pressures can be high. The subtle transition from granitic to pegmatitic texture so common at Spruce Pine (and in other deep-seated systems) was probably related to the development of zones of water-enriched magma and delayed nucleation in the hydrous melts. Deformation (evidenced by mortar texture) of the Spruce Pine Plutonic Suite was related to later regional deformation.

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“…The type of isograd (Equation (1a, b)) in the southern part ( Figure 3) has been well documented in the southern Appalachian Mountains (Abbott 1982;Abbott and Raymond 1984;McSween et al 1989), where it can be related to Barrovian facies-series mineral assemblages in closely associated pelitic rocks, as illustrated in Figure 4. Reactions marked by fine lines in Figure 4 refer to pelitic compositions in the AFM projection (Abbott et al 1996).…”

Section: Westphalia Schistmentioning

confidence: 99%

Amphibolite and blueschist–greenschist facies metamorphism, Blue Mountain inlier, eastern Jamaica

Abbott

Bandy

2008

Geological Journal

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Cretaceous (possibly older) metamorphic rock occurs mainly in the Blue Mountain inlier in eastern Jamaica. Fault-bounded blocks reveal two styles of metamorphism, Westphalia Schist (upper amphibolite facies) and Mt. Hibernia Schist (blueschist (BS)-greenschist (GS) facies). Both Westphalia Schist and Mt. Hibernia Schist preserve detailed records of retrograde P-T paths. The paths are independent, but consistent with different parts of the type-Sanbagawa metamorphic facies series in Japan. For each path, phase relationships and estimated P-T conditions support a two-stage P-T history involving residence at depth, followed by rapid uplift and cooling. Conditions of residence vary depending on the level in a tectonic block. For the critical mineral reaction (isograd) in Westphalia Schist, conditions were P $7.5 kbars, T $6008C (upper amphibolite facies). Retrograde conditions in Hibernia Schist were P ¼ 2.6-3.0 kbars, T ¼ 219-2378C for a(H 2 O) ¼ 0.8-1.0 (GS facies). Mt. Hibernia Schist may represent a volume of rock that was separated and uplifted at an early time from an otherwise protracted P-T path of the sort that produced the Westphalia Schist. Reset K-Ar ages for hornblende and biotite indicate only that retrograde metamorphism of Westphalia Schist took place prior to 76.5 Ma (pre-Campanian). Uplift may have commenced with an Albian-Aptian ($112 Ma) orogenic event.

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The Ashe Metamorphic Suite, Northwest North Carolina; metamorphism and observations on geologic history

Cited by 42 publications

References 0 publications

The Ocean – Continent Transition Zones Along the Appalachian – Caledonian Margin of Laurentia: Examples of Large-Scale Hyperextension During the Opening of the Iapetus Ocean

The Ocean – Continent Transition Zones Along the Appalachian – Caledonian Margin of Laurentia: Examples of Large-Scale Hyperextension During the Opening of the Iapetus Ocean

Mineralogy and petrogenesis of pegmatites in the Spruce Pine District, North Carolina, USA

Amphibolite and blueschist–greenschist facies metamorphism, Blue Mountain inlier, eastern Jamaica

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