Metamorphic discontinuities in the Pennsylvania – Delaware Piedmont: evidence for early Paleozoic assembly

Interpretation of the early Paleozoic tectonic history of the Appalachian Piedmont is commonly complicated by post-Taconian deformation that has disrupted and obscured early Paleozoic structure. Blocks that lie between zones where later deformation has been concentrated, however, may preserve evidence of Taconian high-grade metamorphism. In these cases, analysis of metamorphism allows an interpretation of the early Paleozoic tectonic history of the block and its surrounding region. A block of this type in the Pennsylvania–Delaware Piedmont includes three tectonic units: (1) the basement-cored massifs of southeastern Pennsylvania with Grenvillian gneiss unconformably overlain by the Setters Formation and Cockeysville Marble; (2) the Wilmington Complex arc terrane; and (3) the Wissahickon Group and associated metaigneous rocks. A metamorphic gradient and inferred temperature inversion within the Wissahickon Group and a metamorphic discontinuity at the Wissahickon Group–Cockeysville Marble contact are used to establish that the Wilmington Complex was first joined with the Wissahickon Group and that the two units were then emplaced together onto the rocks now exposed in the massifs. An observed correlation of amphibolite-facies mineral assemblages in the Cockeysville Marble with their distance from the Wissahickon Group indicates that peak metamorphism of the marble occurred after the metapelites were emplaced. As amphibolite-facies metamorphism of the basement-cored massifs has been previously dated as Taconian, it follows that the assembly of these tectonic units occurred in the early Paleozoic.

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Early Paleozoic composite mélange terrane, central Appalachian Piedmont, Virginia and Maryland; Its origin and tectonic history

Ménges Olistostromes of the U.S. Appalachians - Geological Society of America Special Papers ◽

10.1130/spe228-p135 ◽

1989 ◽

pp. 135-194 ◽

Cited By ~ 7

Author(s):

Louis Pavlides

Keyword(s):

Early Paleozoic ◽

Tectonic History ◽

Appalachian Piedmont

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The early Archaean to Proterozoic history of the Isukasia area, southern West Greenland

Bulletin Grønlands Geologiske Undersøgelse ◽

10.34194/bullggu.v154.6696 ◽

1986 ◽

Vol 154 ◽

pp. 1-80

Author(s):

A.P Nutman

Keyword(s):

Volcanic Rocks ◽

Amphibolite Facies ◽

Iron Formation ◽

Banded Iron Formation ◽

Stratigraphic Sequence ◽

Supracrustal Belt ◽

History Of ◽

High Grade Metamorphism ◽

Polyphase Metamorphism ◽

Chemical Sediments

The c. 3800 Ma Isua supracrustal belt and associated smaller bodies of supracrustal rocks are intruded by >3600 Ma orthogneisses. A coherent stratigraphic sequence is recognised consisting of interlayered metabasic rocks, metasediments derived from volcanic rocks, chemical sediments, and metabasic and ultramafic intrusions. Despite repeated deformation and high-grade metamorphism sedimentary structures are locally preserved. The depositional environment was probably an immersed volcanic region remote from areas of significantly older crust. Conglomeratic structures in a metachert and banded iron formation unit suggest shoaling and shallow water conditions. Felsic sediments locally preserve evidence of deposition from turbidite flows. The Isua supracrustal rocks are regarded as thin fragments of a thicker, more extensive sequence. The orthogneisses that intrude the supracrustal rocks consist of 3750-3700 Ma multiphase tonalites (the grey gneisses) which were first intruded by the basic Inaluk dykes, then by abundant shallow-dipping swarms of c. 3600 Ma granite sheets (the white gneisses) and finally by c. 3400 Ma pegmatitic gneiss sheets. These early Archaean rocks were metamorphosed under amphibolite facies conditions and repeatedly deformed prior to intrusion of the Tarssartôq basic dykes in the mid Archaean. In the late Archaean (3100-2500 Ma) there was polyphase metamorphism up to amphibolite facies grade and two or more stages of deformation and local intrusion of granitic gneiss sheets and pegmatites. However, despite general strong deformation there is a large augen of low deformation preserved within the arc of the Isua supracrustal belt. During the Proterozoic there was intrusion of basic dykes, major faulting with associated recrystallisation under uppermost greenschist to lowermost amphibolite facies conditions, followed by heating and intrusion of acid dykes at c. 1600 Ma. No profitable mineralisations have been located.

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Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern Norway

10.5194/egusphere-egu21-7179 ◽

2021 ◽

Author(s):

Stephan Höpfl ◽

Jiří Konopásek ◽

Holger Stünitz ◽

Steffen G. Bergh

Keyword(s):

The Arctic ◽

Low Grade ◽

Prograde Metamorphism ◽

Northern Norway ◽

Metamorphic History ◽

Scandinavian Caledonides ◽

Nappe Complex ◽

History Of ◽

Peak Metamorphism ◽

Metamorphic Gradient

Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern NorwayH&#246;pfl Stephan1, Konop&#225;sek Ji&#345;&#237;1, St&#252;nitz Holger1,2 Bergh G., Steffen1UiT Norges arktiske universitet, Institutt for geovitenskap, [email protected]&#160;1Department of Geosciences, UiT The Arctic University of Norway, Troms&#248; 9037, Norway2Institut des Sciences de la Terre (ISTO), Universit&#233; d&#8217;Orleans, Orleans 45100, France&#160;The Balsfjord Series is located in the central part of Troms&#8211;Finnmark County, northern Norway, and is part of the upper allochthon of the Scandinavian Caledonides. It consists of an Ordovician&#8211;Silurian metsedimentary sequence lying on top of the mostly gabbroic Lyngen Magmatic Complex (LMC). The unit exhibits an inverted metamorphic gradient, where the metamorphic conditions increase from the base to the top, from very low grade in the southeast to medium grade in the west and northwest. The Balsfjord Series is sandwiched between two high-grade units, the Nakkedal + Troms&#248; Nappe Complex in the hanging wall and the Nordmannvik Nappe as the top part of the Reisa Nappe Complex (RNC) in the footwall. The Nakkedal + Troms&#248; Nappe Complex features metamorphic peak ages of ca. 455&#8211;450 Ma and the Nordmannvik Nappe of ca. 430 Ma. The peak metamorphism of the Balsfjord Series has never been dated and the role of the inverted metamorphic gradient is not yet understood. One of the main motivations in this project is to resolve the Caledonian deformation history in the Balsfjord Series, ideally leading to a regional tectonic model explaining the tectonostratigraphic and metamorphic relationships between the abovementioned units.The Balsfjord Series features two main discernible folding phases. The earlier phase displays tight to isoclinal folds with flat lying axial surfaces parallel to the penetrative foliation. Observed fold axes are parallel with the stretching lineation. These folds are best preserved in the northwestern, upper part of the unit and are syn-metamorphic in certain areas, as they fold original bedding (transposed foliation). A later folding phase is represented by mainly open folds with inclined to steep axial surfaces. Their fold axes are gently plunging with a predominant NE&#8211;SW orientation. We interpret these two folding events to be genetically related but slightly diachronous. The earlier folding phase with flat lying axial surfaces was likely generated during nappe thrusting and peak metamorphism of the Balsfjord Series. The subsequent open folding with inclined to steep axial surfaces is explained as a result of continued shearing and shortening of the weaker metapelitic Balsfjord Series against the more rigid gabbroic part of the LMC during the late stages of the Caledonian nappe thrusting.&#160;&#160;&#160;&#160;&#160;&#160;Observed thrust kinematics and penetrative retrogression at the bottom of the Nakkedal + Troms&#248; Nappe Complex suggest that its final exhumation took place during prograde metamorphism of the underlying Balsfjord Series. The ongoing dating of the prograde metamorphism in the Balsfjord series will provide important information about a possible continuity between the timing of peak metamorphism in the Nakkedal + Troms&#248; Nappe Complex, the Balsfjord series and the underlying RNC.

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