130 Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part II

This session explores the evolution of orogenesis on Earth, encompassing applications of sedimentology, petrology, geochemistry, paleomagnetism, geochronology, and structural geology. We encourage studies combining field observations with laboratory and modeling techniques to unravel tectonic processes.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 217C

Neodymium Isotope Constraints on Tectonic Models for the Southernmost Appalachians, Alabama

Paper number: 130-9
Program: Technical Sessions
Time: 3:35 PM – 3:50 PM

Authors

Abstract

Much of the Blue Ridge in Alabama, Georgia, and South Carolina has been interpreted as metamorphosed components of an Ordovician back-arc basin. To better constrain this model, we report new Nd isotope data for greenstone and amphibolite in the western (Talladega Group) and eastern (Ashland Supergroup) Blue Ridge, Alabama. 

Three Hillabee Greenstone (HD), two Mitchell Dam amphibolite (MD), and one Ketchepedrakee amphibolite (KA) samples are basalt (N=5) or basaltic andesite (N=1) based on total alkalis vs. silica. The greenschist facies HD rocks are massive actinolitic amphibole, albite-rich plagioclase, white mica, and quartz. The amphibolite facies MD and KA rocks are foliated hornblende and oligoclase ± quartz. HD and MD samples have a narrow range of REE contents with Sm = 2.4 to 3.0 and Nd = 7.3 to 9.7 ppm. However, the KA sample has higher REE with 5.1 and 17.7 ppm Sm and Nd, respectively. 147Sm/144Nd values for all samples are 0.17 to 0.20. Initial epsilon Ndinitial at 470 Ma (eNd) values are in a narrow range of 2.4 to 4.5.

This narrow range of the new eNd values is similar to the proposed HG correlative Pumpkinvine Creek Formation northwest of Atlanta. However, those rocks have a greater eNd range (N=16) including 4 evolved (-0.5 to -3.2) and 7 values less than AL rocks. Additional samples are needed to confirm if the HG has a similar range to the Pumpkinvine Creek Formation. All of the Alabama samples have primitive eNd values slightly above the present chondrite model mantle value. We infer that HG-MD-KA samples have igneous protoliths and were sourced from the upper mantle. The new whole rock major and trace element compositions, and Nd isotope data are compatible with a back-arc basin interpretation with only limited interaction between magmas and evolved crust. Limited age constraints suggest that the metaigneous rocks and associated metasediments were likely emplaced/deposited between 480 and 460 Ma. All of these rocks were subsequently metamorphosed between 360 and 320 Ma; therefore, extension and magmatism must have ceased by ca. 360 Ma.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414136

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