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 Mark G. Steltenpohl Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA Paul M. Mueller Ann L. Heatherington Department of Geology, University of Florida, Gainesville, Florida 32611, USA Thomas B. Hanley Department of Chemistry and Geology, Columbus State University, Columbus, Georgia 31907, USA Joseph L. Wooden U.S. Geological Survey, Menlo Park, California 94025, USA ABSTRACT Uchee terrane may be correlative with meta- most signifi cant events in Appalachian history morphic basement of the Suwannee terrane. (Hibbard and Sampson, 1995; Hibbard, 2000; The poorly known, suspect, Uchee ter- The ca. 300 Ma overgrowths on zircons are Hibbard et al., 2002). The collage of suspect rane occupies a critical tectonic position with compatible with previously reported 295 to and accreted Gondwanan/peri-Gondwanan regard to how and when peri-Gondwanan 288 Ma 40Ar/39Ar hornblende dates on Uchee terranes within the Carolina zone constitutes (Carolina) and Gondwanan (Suwannee) ter- terrane rocks, which were interpreted to indi- a major part of the Appalachian orogen (Fig. ranes were sutured to Laurentia. It lies sand- cate deep tectonic burial of the Uchee terrane 1). A paucity of reliable isotopic dates within wiched between Laurentian(?) continental contemporaneous with the Alleghanian orog- many of the terranes makes their pre-Appala- basement exposed in the Pine Mountain eny recorded in the foreland. Temperature- chian relations uncertain and, together with window and adjacent buried Gondwanan time paths for the Uchee terrane are similar limited fi eld/structural control, explains why crust of the Suwannee terrane. The Uchee to that of the Pine Mountain terrane, indicat- the time of docking remains a debatable issue. terrane has been proposed as both a septum ing a minimum age of ca. 295 Ma for dock- The Carolina zone comprises heterogeneously of Piedmont rocks that once was continuous ing. In terms of tectono-metamorphic history deformed and metamorphosed Neoproterozoic across the erosionally breached Pine Moun- of the Uchee terrane, it is important to note to Cambrian magmatic arc terranes that formed tain window or part of the Carolina zone. To that no evidence for intermediate “Appala- prior to the Iapetus ocean, likely off the margin help resolve this issue, we conducted U-Pb chian” dates (e.g., Acadian or Taconian) has of Gondwana (Hibbard and Sampson, 1995; (SHRIMP-RG) (sensitive high-resolution ion been reported. This younger history, together Hibbard, 2000; Hibbard et al., 2002). Dock- microprobe–reverse geometry) zircon studies with the ages of metaigneous rocks and evi- ing has been proposed to have occurred during and whole-rock isotopic analyses of principal dence for pre-Grenville basement, suggests the Taconic (Carolina zone subducted beneath metasedimentary and metaplutonic units. the Uchee terrane is likely of Gondwanan Laurentia; Hibbard, 2000), Acadian (Laurentia U-Pb ages for zircons from the Phenix City origin and may be related to Carolina zone beneath Carolina zone; Wortman et al., 1998; Gneiss suggest igneous crystallization at ca. terranes that accreted during the Allegha- Hatcher et al., 1999; Bream et al., 2000, 2004; 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, nian orogeny. Merschat et al., 2005; Hatcher and Merschat, and a ca. 300 Ma (Alleghanian) overprint 2006), or as late as the early Alleghanian (Den- recorded by zircon rims. Zircons from the Keywords: Uchee terrane, Carolina zone, nis and Wright, 1997; West, 1998). metasedimentary/metavolcaniclastic Moffi ts Gondwana, peri-Gondwana, southern Appala- The poorly known Uchee terrane occupies a Mill Schist yield bimodal dates at ca. 620 and chians critical tectonic position with regard to how and 640 Ma. The 620 to 640 Ma dates make these when the Carolina zone was sutured to Lauren- rocks age-equivalent to the oldest parts of the INTRODUCTION tia (Fig. 1). It lies sandwiched between prob- Carolina slate belt (Virgilina and Savannah able Laurentian continental basement exposed River) and strongly suggest a Gondwanan The nature and timing of docking of the in the Pine Mountain window and overlying (Pan-African and/or Trans-Brasiliano) ori- Carolina zone to Laurentia is controversial Gondwanan crust of the Suwannee terrane, gin for the Uchee terrane. Alternatively, the and remains one of the least understood but which is buried beneath sediments of the Gulf Geosphere; February 2008; v. 4; no. 1; p. 131–144; doi: 10.1130/GES00079.1; 5 fi gures; 3 tables. For permission to copy, contact [email protected] 131 © 2008 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/1/131/3339377/i1553-040X-4-1-131.pdf by guest on 28 September 2021 Steltenpohl et al. Figure 1. (A) Tectonic map of the southern Appalachians with section line A–A' (modifi ed from: Hatcher, 2004; Morton et al., 1984, 1989; Hibbard et al., 2002, 2006; Steltenpohl, 2005a). Red square is area of detailed geologic map in Figure 2. (B) Cross section A–A' (modifi ed from: Thomas and coworkers as presented in Hatcher et al., 1990; Steltenpohl 2005a). 132 Geosphere, February 2008 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/1/131/3339377/i1553-040X-4-1-131.pdf by guest on 28 September 2021 Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and Georgia Coastal Plain (Fig. 1). In general, two contrast- from Alabama and westernmost Georgia (Bent- be coeval with these orthogneiss sheets. Dates ing schools of thought have held sway for the ley and Neathery, 1970; Hanley, 1983, 1987; of crystallization for neither the Motts Gneiss origin of the Uchee terrane: (1) Laurentian— McRae, 1992; McRae and Steltenpohl, 1993; nor the Hospilika Granite are known. that is, it may be a septum of Piedmont rocks Steltenpohl and Salpas, 1993; Hanley and Fabric relations in some rocks of the Uchee that once was continuous across the now ero- Steltenpohl, 1997; Steltenpohl, 2005a, 2005b). terrane clearly indicate that two amphibolite- sionally breached Pine Mountain anticlinorium These studies indicate that the Uchee is divis- facies events affected them (McRae, 1992; (Hanley et al., 1997); or (2) Exotic—that is, it ible into three parts (Fig. 2): a structurally lower McRae and Steltenpohl, 1993). The dominant might be part of the peri-Gondwanan Carolina dioritoidal gneiss and amphibolite complex gneissosity/schistosity, S0/S1, is interpreted as a zone (Russell, 1978, 1985; Hooper and Hatcher, (Phenix City Gneiss; Figs. 3A and 3B); an inter- transposition foliation formed under uppermost 1988, 1990; Hanley et al., 1997; Hibbard et al., mediate-level package of relatively well-lay- amphibolite-facies conditions (630–780 °C and 2002; McBride et al., 2005) or the Gondwanan ered metasedimentary and metavolcanic rocks 5.7–10.6 kbar; Chalokwu, 1989) during M1; Suwannee terrane. Neither the strongly meta- (Moffi ts Mill Schist; Fig. 3D); and an upper S0 is interpreted as an earlier primary layering morphosed lithologies nor the existing sparse complex of diverse migmatite, orthogneiss, and (bedding or other type of compositional band- isotopic age information, however, allows for amphibolite (North Columbus metamorphic ing) transposed parallel to S1 based on inclu- distinguishing which proposal is correct. Conse- complex; Fig. 3C), all disposed in the core of a sion trails in M1 porphyroblasts, but no other quently, we conducted U-Pb zircon age determi- gently NE-plunging synform (Lake Oliver syn- evidence for S0 was clearly observed. M2 and nations to place more defi nitive age control on form; Fig. 2). Migmatites are commonplace but D2 resulted in a mostly weak but locally intense the times of protolith formation of metaigneous occur randomly at all structural levels in each of schistosity and/or mineral lineation (S2 and/or and metasedimentary rocks and timing of meta- the three rock packages. Minor ultramafi c rocks L2, respectively) that clearly overprint S0/S1 and morphism of rocks in the Uchee terrane. Our occur together with the mafi c units. Major- and all earlier formed structures (Figs. 3B, 3C, and results indicate that an exotic (non-Laurentian) trace-element geochemistry indicates a volcanic 3F). Retrograde assemblages defi ning S2 and L2 origin for the Uchee terrane is most probable, arc to backarc setting (Chalokwu and Hanley, and symplectic overgrowths formed under mid- thus providing a new puzzle piece helping to 1990; Steltenpohl et al., 2002; Hanley et al., dle-amphibolite-facies conditions (550–580 °C constrain Carolina zone tectonic evolution and 2005). Limited U-Pb dates on multigrain zircon and 6.8–7.6 kbar; Chalokwu, 1989). Steltenpohl its docking with Laurentia. populations and Rb-Sr errorchron dates on some and Kunk (1993) reported 40Ar/39Ar hornblende of the Uchee rocks were previously reported to cooling dates from rocks of the Uchee terrane GEOLOGIC CONTEXT range from Neoproterozoic to Devonian (Rus- that document amphibolite-facies conditions sell, 1978; Maher et al., 1992). (i.e., ~500 °C closure temperature for horn- The Uchee terrane is the most eastern and Two suites of late-stage granitoidal plutonic blende) lasting as late as 288 Ma, recording the internal terrane exposed in the Alabama and rock intrude the Uchee protoliths (Fig. 2). late stages of the Alleghanian event. central Georgia Piedmont (Fig. 1). It is bounded The earlier phase, the Motts Gneiss (Bentley The Pine Mountain terrane (Figs. 1 and 2) on the northwest by fundamental southern and Neathery, 1970; Raymond et al., 1988), comprises multiply folded and faulted Grenville Appalachian mylonite zones. The Bartletts occurs as thick (up to 3 km), extensive (up to (ca. 1.05 Ga) basement gneisses and younger Ferry, Goat Rock, and Box Ankle fault zones 30 km strike length), tabular sheets (Fig.