Chronology of Paleozoic metamorphism and deformation in the Blue Ridge thrust complex, North Carolina and Tennessee

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ID: 298713
1997
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Abstract
The Blue Ridge province in northwestern North Carolina and northeastern Tennessee records a multiphase collisional and accretionary history from the Mesoproterozoic through the Paleozoic. To constrain the tectonothermal evolution in this region, radiometric ages have been determined for 23 regionally metamorphosed amphibolites, granitic gneisses, and pelitic schists and from mylonites along shear zones that bound thrust sheets and within an internal shear zone. Samples were collected along a northwest-southeast traverse across three crystalline thrust sheets located west of the Grandfather Mountain window. Ordovician metamorphic mineral ages (Llanvirnian to Caradocian) are recorded in the highest thrust sheets within the Blue Ridge thrust complex. Six samples of metapelite and amphibolite from the Spruce Pine thrust sheet and five samples of basement gneiss and metagabbro from the Pumpkin Patch thrust sheet yield Sm-Nd and Rb-Sr garnet ages of 460 to 450 Ma and hornblende ages of 472 to 451 Ma. In addition, multiple mineral isochron ages of 457+ or -2 and 458+ or -17 Ma were obtained from a Pumpkin Patch metagabbro. The garnet ages from the Pumpkin Patch thrust sheet (458, 455, and 451 Ma) are similar to those from the structurally overlying Spruce Pine thrust sheet (460, 456, 455, 455, and 450 Ma). Both thrust sheets exhibit similar upper amphibolite-facies conditions. Because of the high closure temperature for garnet, the garnet ages are interpreted to date growth at or near the peak of Taconic metamorphism. Devonian metamorphic ages are recognized in the Spruce Pine thrust sheet, where Sm-Nd and Rb-Sr garnet ages of 386 and 393 Ma and mineral isochron ages of 397+ or -14 and 375+ or -27 Ma are preserved. Hornblendes record similar ^40^ Ar/ ^39^ Ar, Sm-Nd, and Rb-Sr ages of 398 to 379 Ma. Devonian ^40^ Ar/ ^39^ Ar hornblende ages are also recorded in the structurally lower Pumpkin Patch thrust sheet. The Devonian mineral ages are interpreted to date a discrete tectonothermal event, as opposed to uplift and slow cooling from an Ordovician metamorphic event. This interpretation is based in part on the fact that Devonian and Ordovician mineral ages are intermixed spatially within the Spruce Pine and Pumpkin Patch thrust sheets. ^40^ Ar/ ^39^ Ar muscovite ages from regionally metamorphosed rocks indicate that much of the Blue Ridge cooled slowly to temperatures of about 375 degrees C by the Late Mississippian. White micas from mylonite zones separating thrust sheets record ^40^ Ar/ ^39^ Ar plateau ages of 323, 327, 328, and 336 Ma and Rb-Sr ages of 324, 324, 326, and 326 Ma. These reflect the age of Late Mississippian mylonitization (mica growth below or near respective Sr and Ar closure temperatures). The Mississippian mylonitization is interpreted to represent thrusting and initial assembly of crystalline sheets associated with the Alleghanian orogeny. The composite thrust stack of the Blue Ridge complex was subsequently thrust northwestward along the Linville Falls fault during middle Alleghanian orogeny (about 300 Ma).
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Authors Steven A. Goldberg, R. D. Dallmeyer
Journal american journal of science
Year 1997
DOI
10.2475/ajs.297.5.488
URL
https://ajs.scholasticahq.com/article/60755.pdf
Keywords Keywords not found

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