Geologic Investigations in the Smith River Allochthon and Lynchburg Group, Southern Appalachians: Implications for the Evolution of the Eastern Laurentian Margin

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Title: Geologic Investigations in the Smith River Allochthon and Lynchburg Group, Southern Appalachians: Implications for the Evolution of the Eastern Laurentian Margin
Author: Carter, Brad T
Advisors: Gerald S. Janowitz, Committee Member
David J. DeMaster, Committee Member
Ronald V. Fodor, Committee Member
Edward F. Stoddard, Committee Chair
Robert J. Tracy, Committee Member
Abstract: Detrital zircon age data from the SRA range from 0.9-1.5 Ga and is strikingly similar to the 0.9-1.4 Ga ages obtained from the Lynchburg Group. The meta-mafic rocks of the SRA are geochemically similar to those of the Lynchburg Group and based on whole-rock compositions, both suites are interpreted as continental rift basalts. These geochronological and geochemical data imply that the SRA originated in a rift basin along the Laurentian margin. Thus the SRA-Lynchburg Group contact likely represents closure of a Laurentian rift basin⁄seaway. The tectonothermal histories of the SRA and Lynchburg Group were deciphered by integrating structural and petrographic observations with MnNaCaKFMASHTi phase equilibria modeling, garnet growth P-T estimates, and EPMA monazite geochronology. The SRA was involved in four tectonothermal events before it was emplaced upon the Lynchburg Group. The oldest event occurred at ca. 530 Ma and is interpreted as a rifting event that led to the opening of a seaway along the Laurentian margin. The second event occurred at ca. 500 Ma, but the nature of this event is unclear. The third event, ca. 460 Ma, occurred under P-T conditions of 580 C and 4 kb. The final event before emplacement occurred at 430-440 Ma and resulted in greenschist to granulite static metamorphism. The first tectonothermal event shared by the SRA and Lynchburg Group occurred ca. 425 Ma, thus constraining the timing of juxtaposition of the SRA and Lynchburg Group via the Ridgeway fault. This event was also the first event that affected the Lynchburg Group. Greenschist facies retrogression is evident in SRA rocks within the Ridgeway fault zone. Whereas the upper Lynchburg Group experienced greenschist facies metamorphism, the lower Lynchburg Group was progressively metamorphosed up to amphibolite facies and exhibits Barrovian-type garnet growth P-T paths, from 500-560 C, 5-6.5 kb to 620-640 C, 8.8-9.3 kb. Both terranes were later involved in an upper-crustal event roughly constrained in age from 390 to 345 Ma.
Date: 2006-07-16
Degree: PhD
Discipline: Marine, Earth and Atmospheric Sciences
URI: http://www.lib.ncsu.edu/resolver/1840.16/5263


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