Isotopic Age Constraints and Metamorphic History of the Talladega Belt: New Evidence for Timing of Arc Magmatism and Terrane Emplacement along the Southern Laurentian Margin

Abstract

Talladega belt, southernmost Appalachians, provide insight into the timing and nature of pre-Alleghanian tectonism in this region. Low-grade metasedimentary rocks of the Talladega belt represent the outermost preserved portions of the southern Laurentian margin, thus recording the earliest orogenic events that affected the margin, in addition to later overprinting events. These rocks are structurally overlain by the Hillabee Greenstone metavolcanic sequence, from which metadacite yields an age of ‫4ע074‬ Ma (ion microprobe U-Pb zircon age). Hillabee geochemistry indicates formation in an arc or back-arc setting. We suggest correlation of the Hillabee Greenstone with 460-470-Ma arc-related rocks in the Dahlonega gold belt, which extends along strike with the Hillabee through Georgia and into North Carolina, and interpret the Hillabee as the southernmost volcanic expression of an Early-Middle Ordovician arc that formed outboard of Laurentia. Rocks of the Talladega belt, including the Hillabee, preserve a record of only one Paleozoic dynamothermal metamorphic event. 40 Ar/ 39 Ar dates are consistent with metamorphism following deposition of the youngest biostratigraphically dated unit, the Erin Slate, which contains early Mississippian Periastron plant fossils (360-350 Ma). Muscovite (closure temperature 350Њ-400ЊC) yields internally consistent ages between 334 and 320 Ma. Metamorphism of the Talladega belt, therefore, is constrained to the interval between 360 and 320 Ma. The pre-to synmetamorphic thrust contact between the Hillabee Greenstone and Talladega belt metasedimentary rocks must have formed after deposition of the Erin Slate at 360-350 Ma but before metamorphism of the Talladega belt (no later than ∼320 Ma). We propose that the Ordovician Hillabee-Dahlonega arc terrane first collided with the Laurentian continental margin between 360 and 320 Ma, with variations in timing of deformation and metamorphic character along the collision zone.

Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and Georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

“…Steltenpohl et al (2004) report a 354 ± 140 Ma lower intercept U-Pb age on detrital zircons from the Hollis Quartzite, which is consistent with fossil evidence for post-early Mississippian metamorphism in the nearby Talladega slate belt ( Fig. 2; Gastaldo et al, 1993;McClellan et al, 2007) and a <337 Ma 40 Ar/ 39 Ar date suggested for hornblende from the Pine Mountain window (Steltenpohl and Kunk, 1993). Nonetheless, the T-t path corroborates the suggestion of Steltenpohl and Kunk (1993) that the Pine Mountain terrane had occupied middle-crustal levels during the Alleghanian event and that it was joined with the Uchee terrane either prior to or during this event before being uplifted and cooled as a somewhat coherent block.…”

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“…Based on decades of surface geological studies, correlations of buried rocks with otherwise known features outside of Alabama, and drill-core data, the crust underlying Alabama is recognized as belonging to three fundamental types: 1) Laurentian, 2) peri-Gondwanan, and 3) Gondwanan (Neathery and Thomas, 1975;Hatcher, 1978Hatcher, , 1987Hatcher, , 2010Tull, 1982;Chowns and Williams, 1983;Horton et al, 1984;Tull, 1978Tull, , 1980Tull, , 1982Tull, , 1984Guthrie and Raymond, 1992;Drummond et al, 1994Drummond et al, , 1997Steltenpohl, 2005;McClellan et al, 2005McClellan et al, , 2007Steltenpohl et al, 2008Steltenpohl et al, , 2010a. Our results from comparing regional geophysical data with the surface and subsurface geology indicate clear demarcation of Laurentian crust in roughly the northwestern half of Alabama, and periGondwanan or Gondwanan crust underlying the southeastern quarter.…”

“…Farther northeast into Georgia most geologic maps depict the fault to be the Allatoona fault (Hurst, 1973;Tull, 1978;Hatcher et al, 1990;Tull and Holm, 2005). The Goodwater-Enitachopco/Allatoona fault has been interpreted as a southeast-dipping reverse (thrust) fault (Neathery and Tull, 1975;Pickering, 1976;Tull, 1978Tull, , 1984Costello, 1980, 1982;Crawford and Cressler, 1982;McConnell and Abrams, 1984;Osborne et al, 1988;McClellan et al, 2005;Tull and Holm, 2005). , on the other hand, interpreted the Goodwater-Enitachopco to be a normal fault, reasoning that a thrust fault cutting the basal eastern Blue Ridge thrust (i.e., the Hollins line) should emplace Talladega slate belt rocks in the hanging wall.…”

“…Blue Ridge rocks of the Talladega slate belt consist of Cambrian to early Mississippian clastic and carbonate rocks, which are overthrust from the southeast by the Hollins line fault (Butts, 1926;Tull, 1982;Tull et al, 1988;Gastaldo et al, 1993). The Hillabee and Hollins line faults thrust the Ordovician Hillabee Greenstone (dominantly mafi c but containing some felsic volcanic rocks) and overlying distal Laurentian clastic metasedimentary and metavolcanic rocks of the eastern Blue Ridge over all of the tectonic units to the west (Tull, 1978(Tull, , 1980(Tull, , 1982(Tull, , 1984(Tull, , 1995Tull et al, 2007;McClellan et al, 2005McClellan et al, , 2007. These faults may be a suture equivalent to the Allatoona-Hayesville-Soque River fault system farther northeast.…”

“…Russell (1978) reported whole-rock Rb-Sr "errorchron" ages for the Kowaliga and Zana granites within the Silurian and Devonian Periods, respectively, whereas U-Pb analysis of zircons (multi-grain aliquots) gave Middle Ordovician ages. Most workers interpret the eastern Blue Ridge of Alabama to be part of the Neoproterozoic slope-rise facies of the distal Laurentian margin (Drummond et al, 1994(Drummond et al, , 1997Steltenpohl, 2005;McClellan et al, 2007;Tull et al, 2007).…”