Abstract

The poorly known, suspect, Uchee terrane occupies a critical tectonic position with regard to how and when peri-Gondwanan (Carolina) and Gondwanan (Suwannee) terranes were sutured to Laurentia. It lies sandwiched between Laurentian(?) continental basement exposed in the Pine Mountain window and adjacent buried Gondwanan crust of the Suwannee terrane. The Uchee terrane has been proposed as both a septum of Piedmont rocks that once was continuous across the erosionally breached Pine Mountain window or part of the Carolina zone. To help resolve this issue, we conducted U-Pb (SHRIMP-RG) (sensitive high-resolution ion microprobe–reverse geometry) zircon studies and whole-rock isotopic analyses of principal metasedimentary and metaplutonic units. U-Pb ages for zircons from the Phenix City Gneiss suggest igneous crystallization at ca. 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, and a ca. 300 Ma (Alleghanian) overprint recorded by zircon rims. Zircons from the metasedimentary/metavolcaniclastic Moffits Mill Schist yield bimodal dates at ca. 620 and 640 Ma. The 620 to 640 Ma dates make these rocks age-equivalent to the oldest parts of the Carolina slate belt (Virgilina and Savannah River) and strongly suggest a Gondwanan (Pan-African and/or Trans-Brasiliano) origin for the Uchee terrane. Alternatively, the Uchee terrane may be correlative with metamorphic basement of the Suwannee terrane. The ca. 300 Ma overgrowths on zircons are compatible with previously reported 295 to 288 Ma 40Ar/39Ar hornblende dates on Uchee terrane rocks, which were interpreted to indicate deep tectonic burial of the Uchee terrane contemporaneous with the Alleghanian orogeny recorded in the foreland. Temperature-time paths for the Uchee terrane are similar to that of the Pine Mountain terrane, indicating a minimum age of ca. 295 Ma for docking. In terms of tectono-metamorphic history of the Uchee terrane, it is important to note that no evidence for intermediate “Appalachian” dates (e.g., Acadian or Taconian) has been reported. This younger history, together with the ages of metaigneous rocks and evidence for pre-Grenville basement, suggests the Uchee terrane is likely of Gondwanan origin and may be related to Carolina zone terranes that accreted during the Alleghanian orogeny.

INTRODUCTION

The nature and timing of docking of the Carolina zone to Laurentia is controversial and remains one of the least understood but most significant events in Appalachian history (Hibbard and Sampson, 1995; Hibbard, 2000; Hibbard et al., 2002). The collage of suspect and accreted Gondwanan/peri-Gondwanan terranes within the Carolina zone constitutes a major part of the Appalachian orogen (Fig. 1). A paucity of reliable isotopic dates within many of the terranes makes their pre-Appalachian relations uncertain and, together with limited field/structural control, explains why the time of docking remains a debatable issue. The Carolina zone comprises heterogeneously deformed and metamorphosed Neoproterozoic to Cambrian magmatic arc terranes that formed prior to the Iapetus ocean, likely off the margin of Gond-wana (Hibbard and Sampson, 1995; Hibbard, 2000; Hibbard et al., 2002). Docking has been proposed to have occurred during the Taconic (Carolina zone subducted beneath Laurentia; Hibbard, 2000), Acadian (Laurentia beneath Carolina zone; Wortman et al., 1998; Hatcher et al., 1999; Bream et al., 2000, 2004; Merschat et al., 2005; Hatcher and Merschat, 2006), or as late as the early Alleghanian (Dennis and Wright, 1997; West, 1998).

The poorly known Uchee terrane occupies a critical tectonic position with regard to how and when the Carolina zone was sutured to Laurentia (Fig. 1). It lies sandwiched between probable Laurentian continental basement exposed in the Pine Mountain window and overlying Gondwanan crust of the Suwannee terrane, which is buried beneath sediments of the Gulf Coastal Plain (Fig. 1). In general, two contrasting schools of thought have held sway for the origin of the Uchee terrane: (1) Laurentian—that is, it may be a septum of Piedmont rocks that once was continuous across the now erosionally breached Pine Mountain anticlinorium (Hanley et al., 1997); or (2) Exotic—that is, it might be part of the peri-Gondwanan Carolina zone (Russell, 1978, 1985; Hooper and Hatcher, 1988, 1990; Hanley et al., 1997; Hibbard et al., 2002; McBride et al., 2005) or the Gondwanan Suwannee terrane. Neither the strongly metamorphosed lithologies nor the existing sparse isotopic age information, however, allows for distinguishing which proposal is correct. Consequently, we conducted U-Pb zircon age determinations to place more definitive age control on the times of protolith formation of metaigneous and metasedimentary rocks and timing of metamorphism of rocks in the Uchee terrane. Our results indicate that an exotic (non-Laurentian) origin for the Uchee terrane is most probable, thus providing a new puzzle piece helping to constrain Carolina zone tectonic evolution and its docking with Laurentia.

GEOLOGIC CONTEXT

The Uchee terrane is the most eastern and internal terrane exposed in the Alabama and central Georgia Piedmont (Fig. 1). It is bounded on the northwest by fundamental southern Appalachian mylonite zones. The Bartletts Ferry, Goat Rock, and Box Ankle fault zones have juxtaposed the Uchee terrane with Grenville basement and its stratigraphic cover within the Pine Mountain window (Figs. 1 and 2, this paper; Schamel et al., 1980; Sears et al., 1981a; Steltenpohl, 1988; Hooper and Hatcher, 1988; West et al., 1995; McBride et al., 2005). Northeast of the Pine Mountain window, the Modoc fault zone separates the Uchee from the Charlotte terrane (Fig. 2), an infrastructural block within the Carolina zone (Secor et al., 1986; West et al., 1995; Hibbard et al., 2002). The Box Ankle fault is a west-directed thrust (Hooper and Hatcher, 1988; West et al., 1995), whereas the others are right-slip fault zones and each formed at various times during the Middle to Late Carboniferous (Secor et al., 1986; Steltenpohl, 1988; Steltenpohl et al., 1992). To the east, the Uchee terrane is mostly covered beneath sedimentary rocks of the Gulf and Atlantic Coastal Plains. To the northeast, however, it is in tectonic contact with the Milledgeville terrane, a largely unknown suprastructural block within the Carolina zone (Higgins et al., 1988; Hibbard et al., 2002).

The Uchee terrane is not well mapped, and our present understanding is based primarily on M.S. thesis and fieldtrip guidebook reports from Alabama and westernmost Georgia (Bentley and Neathery, 1970; Hanley, 1983, 1987; McRae, 1992; McRae and Steltenpohl, 1993; Steltenpohl and Salpas, 1993; Hanley and Steltenpohl, 1997; Steltenpohl, 2005a, 2005b). These studies indicate that the Uchee is divisible into three parts (Fig. 2): a structurally lower dioritoidal gneiss and amphibolite complex (Phenix City Gneiss; Figs. 3A and 3B); an intermediate-level package of relatively well-layered metasedimentary and metavolcanic rocks (Moffits Mill Schist; Fig. 3D); and an upper complex of diverse migmatite, orthogneiss, and amphibolite (North Columbus metamorphic complex; Fig. 3C), all disposed in the core of a gently NE-plunging synform (Lake Oliver syn-form; Fig. 2). Migmatites are commonplace but occur randomly at all structural levels in each of the three rock packages. Minor ultramafic rocks occur together with the mafic units. Major- and trace-element geochemistry indicates a volcanic arc to backarc setting (Chalokwu and Hanley, 1990; Steltenpohl et al., 2002; Hanley et al., 2005). Limited U-Pb dates on multigrain zircon populations and Rb-Sr errorchron dates on some of the Uchee rocks were previously reported to range from Neoproterozoic to Devonian (Russell, 1978; Maher et al., 1992).

Two suites of late-stage granitoidal plutonic rock intrude the Uchee protoliths (Fig. 2). The earlier phase, the Motts Gneiss (Bentley and Neathery, 1970; Raymond et al., 1988), occurs as thick (up to 3 km), extensive (up to 30 km strike length), tabular sheets (Fig. 2). These granodioritic rocks characteristically are strongly lineated L-tectonites (Fig. 3E). The Hospilika Granite is a later phase occurring as small (<1 km2) vein-like injections that locally intrude the Motts (Fig. 2; McRae, 1992) but do not carry the same intensely developed elongation lineation. The lack of any strongly developed fabric within the Hospilika led workers to hypothesize that they are Carboniferous, late-Alleghanian intrusions (Bentley and Neathery, 1970; McRae, 1992; Steltenpohl and Kunk, 1993). McRae (1992) and Steltenpohl and Kunk (1993) point out several geological relationships that imply the Motts Gneiss protolith might also have intruded during the Carboniferous. First, the Motts Gneiss is sheared into the right-slip Goat Rock fault zone which formed during the late Carboniferous (ca. 288 Ma; Steltenpohl et al., 1992). The elongation lineation within both the Goat Rock fault zone and the Motts Gneiss is coaxial. Second, because the Motts Gneiss is similar in its chemistry, petrology, and field characteristics to Late Carboniferous ortho-gneiss sheets of the Modoc fault zone (ca. 315 Ma; Maher et al., 1992), which occur northeast-ward along strike to the Motts (Fig. 2), it may be coeval with these orthogneiss sheets. Dates of crystallization for neither the Motts Gneiss nor the Hospilika Granite are known.

Fabric relations in some rocks of the Uchee terrane clearly indicate that two amphibolite-facies events affected them (McRae, 1992; McRae and Steltenpohl, 1993). The dominant gneissosity/schistosity, S0/S1, is interpreted as a transposition foliation formed under uppermost amphibolite-facies conditions (630–780 °C and 5.7–10.6 kbar; Chalokwu, 1989) during M1; S0 is interpreted as an earlier primary layering (bedding or other type of compositional banding) transposed parallel to S1 based on inclusion trails in M1 porphyroblasts, but no other evidence for S0 was clearly observed. M2 and D2 resulted in a mostly weak but locally intense schistosity and/or mineral lineation (S2 and/or L2, respectively) that clearly overprint S0/S1 and all earlier formed structures (Figs. 3B, 3C, and 3F). Retrograde assemblages defining S2 and L2 and symplectic overgrowths formed under middle-amphibolite-facies conditions (550–580 °C and 6.8–7.6 kbar; Chalokwu, 1989). Steltenpohl and Kunk (1993) reported 40Ar/39Ar hornblende cooling dates from rocks of the Uchee terrane that document amphibolite-facies conditions (i.e., ∼500 °C closure temperature for hornblende) lasting as late as 288 Ma, recording the late stages of the Alleghanian event.

The Pine Mountain terrane (Figs. 1 and 2) comprises multiply folded and faulted Grenville (ca. 1.05 Ga) basement gneisses and younger stratigraphic cover exposed in a complex tectonic window (Schamel et al., 1980; Sears et al., 1981a; Sears and Cook, 1984; Hooper and Hatcher, 1988). A platformal metasedimentary cover sequence mostly mantles the Grenville gneisses and is called the Pine Mountain Group (Galpin, 1915; Adams, 1933; Crickmay, 1933, 1952). The Pine Mountain Group (Fig. 2) comprises, from stratigraphic bottom to top, the Hollis Quartzite, Chewacla Marble, and Manchester Schist (Crickmay, 1952; Clarke, 1952; Bentley and Neathery, 1970; Raymond et al., 1988), and these units have been suggested to lithologically correlate, respectively, with the Chilhowee-Shady/Knox-Rome platformal sequence in the foreland (Clarke, 1952; Sears et al., 1981b; Steltenpohl, 1992; Yokel et al., 1997). Feldspathic schists lying between the Hollis Quartzite and the basement gneiss, the Halawaka and Sparks schists (Clarke, 1952; Schamel et al., 1980; Raymond et al., 1988), are interpreted as Ocoee rift facies (Clarke, 1952; Bentley and Neathery, 1970; Schamel et al., 1980; Sears et al., 1981a, 1981b; Steltenpohl, 1992; Yokel et al., 1997).

Two distinct metamorphic events affected basement rocks of the Pine Mountain terrane (Schamel and Bauer, 1980; Steltenpohl and Moore, 1988; Steltenpohl, 1992; Steltenpohl and Kunk, 1993). Mesoproterozoic upper amphibolite- to granulite-facies metamorphism, M1, resulted from the Grenville event (Odom et al., 1973, 1985; Steltenpohl and Moore, 1988; Stieve and Size, 1988). Paleozoic (Appalachian) M2 metamorphism is recorded in both the basement gneisses and the Pine Mountain Group and ranged from kyanite and sillimanite grade in Georgia to staurolite grade in Alabama (Sears and Cook, 1984; Steltenpohl and Moore, 1988; Steltenpohl and Kunk, 1993; Yokel, 1996). Timing of M2 is not well constrained because no fossils are reported and only a few reliable isotopic dates exist. Tull (1980) used fossil and isotopic evidence from across the southern Appalachians to infer a widespread Devonian (Acadian) metamorphic peak. Wampler et al. (1970) report a conventional K/Ar mineral cooling date on an actual Pine Mountain rock that was ca. 311 Ma (Alleghanian). 40Ar/39Ar mineral cooling studies corroborated late Pennsylvanian through Permian cooling for the temperature interval between ca. 350 and 180 °C (Steltenpohl and Kunk, 1993). If the lithologic correlations proposed for the Pine Mountain Group cover units are valid, then the metamorphic “peak” post-dated the Early Ordovician. Steltenpohl et al. (2004) reported a ca. 354-Ma lower intercept U-Pb date on zircons from the Hollis Quartzite, but the date carries a large, ±140 Ma error estimate. Steltenpohl and Kunk (1993) reported a “disturbed” 40Ar/39Ar spectrum on hornblende from the basement complex, indicating that cooling through ∼500 °C occurred some time after ca. 337 Ma. In the next section, we report U-Pb isotopic analysis of rutile from the Hollis Quartzite in an attempt to further delimit the higher temperature part of the Pine Mountain cooling path.

U-Pb AND Sm-Nd RESULTS AND INTERPRETATIONS

Analyses of the U-Pb systematics of zircon grains were carried out by sensitive, high-resolution, ion microprobe (SHRIMP), and procedures are the same as those reported in Steltenpohl et al. (2004). For the Nd and Sm analyses, whole-rock powders were dissolved, spiked with a mixed 150Nd/147Sm spike, and Nd and Sm were separated via standard chromatographic methods using laboratories in the Department of Geology at the University of Florida. Analysis of Nd and Sm isotopic composition was performed using a Nu MC-ICP-MS (multi-collector–inductively coupled plasma–mass spectrometer), following methods described by Kamenov (2006). External precision on measured 143Nd/144Nd ratios is ±0.4 epsilon units (2 sigma) based on multiple analysis of the standard JNdi-1. The sample localities (Fig. 2A) and analytical results are presented in 01Tables 1, 2012202, and 033.

Phenix City Gneiss, Uchee Terrane

Sample TH-04 is a quartzofeldspathic (hornblende-plagioclase) gneiss collected near Columbus, Georgia (Fig. 2). U-Pb and Pb-Pb ages of single zircon grains are largely concordant and range from ca. 300 to ca. 1700 Ma, with a strong concentration in the range from 590 to 640 Ma (Fig. 4A). Zircons in this age range have typical igneous Th and U contents (Th/U ratios from 0.5 to 0.8), which suggest the primary protolith to the gneiss was a ca. 600 Ma magmatic (or possibly volcaniclastic) rock. It is difficult to provide a more definitive age estimate, however, due to the scatter of ages near 600 Ma, which may be related to Pb loss associated with a strong, late Paleozoic overprint at ca. 300 Ma or a limited range of magmatic and xenocrystic grains. The late Paleozoic overprint is evident in the relatively large percentage of grains with high-U overgrowths. In most cases, the uranium contents of the overgrowths were so high that useful measurement was not possible. Those that were measurable are characterized by the low Th/U ratios commonly associated with hydrothermal growth; all ages <590 Ma have Th/U <0.1.

A third age component in this rock is evident in zircons with ages in the range of ca. 1000 to ca. 1700 Ma. Th/U ratios for these grains range from 0.1 to 1.0, and the grains are interpreted to be largely, if not completely, igneous in origin. Their tectonic/lithologic origin, however, is less certain because of the overall migmatitic character of the Phenix City Gneiss. If we consider only the relatively homogeneous sample analyzed here, then the most straightforward interpretation is that these >1000 Ma grains are xenocrystic and derived from a chronologically diverse basement into/onto which the protolith was emplaced. It is also possible, however, that these older grains were directly entrained into the melt during anatexis and, therefore, represent the source to some degree. Regardless of the exact method of incorporation, it is important to recognize that these data clearly suggest the presence of older crust within the Uchee terrane. The primary age of the protolith (ca. 600 Ma), in conjunction with the ages of the older xenocrystic zircons, provides a basis for explaining the scatter in the original results of Russell (1985) and for proposing and testing models for correlation of the Uchee terrane to other Appalachian terranes, as described in the discussion section below.

Moffits Mill Schist, Uchee Terrane

Sample UB-1-03 is a quartzofeldspathic schist (metagraywacke) collected from the type locality at Little Uchee Creek near Moffits Mill, Alabama (Figs. 2 and 3D). The age spectrum shows both a traditional histogram of 206Pb/238U ages and the probability distribution derived from these data, which reflect 29 single-grain zircon analyses (SHRIMP-RG) for a single sample of this unit (Fig. 4B). There is a strong concentration of ages at ca. 600 Ma and a range of younger ages down to ca. 300 Ma; grains older than ca. 600 Ma were not detected. Overall, concordance among the <600 Ma grains was less than in A-13 and TH-04; U concentrations range up to 2000 ppm. The data are interpreted to represent deposition of an original sedimentary or volcaniclastic protolith subsequent to ca. 600 Ma, followed by significant metamorphic overprint(s). The limited range of Neoproterozoic ages likely reflects a very limited provenance of the protolith (e.g., a graben within a developing arc). The overprinting is evident exclusively in younger overgrowths (285–375 Ma, mean = 334 Ma), which exhibit the high U/Th ratios (all >20) characteristic of hydrothermally grown zircon. These “Alleghanian” ages were not measured in any whole grains or cores. The two intermediate ages (435 and 483 Ma) exhibit intermediate U/Th ratios and may result from some mixing of older and younger components during analyses.

Migmatized Moffits Mill Schist, Uchee Terrane

U-Pb ages were determined for zircons separated from the leucocratic portion of the migmatitic part of Moffits Mill Schist sample A-13 (Figs. 2 and 3C). Analysis yielded a data set similar to that of Phenix City Gneiss sample TH-04 in terms of a protolith crystallization age of ca. 600 Ma and evidence for late Paleozoic overprinting (cf. Figs. 4A and 4C). The nine most concordant and oldest grains within an older group of 13 grains (585–635 Ma) yielded an error-weighted mean age of 623 ± 7 Ma (2 sigma). This is taken to be the age of emplacement of an igneous protolith of the leucosome. Younger ages recorded from overgrowths were generally late Paleozoic (300–400 Ma). These younger ages are from grains characterized by lower Th/U ratios than the ca. 600 Ma grains and show a general positive correlation of age and Th/U ratio, which suggests that some analyses may have incorporated both overgrowth and original grain. The three analyses with the lowest Th/U (<0.05) are concordant and have 206Pb/238U ages of ca. 300 Ma. The primary difference between this sample and TH-04, therefore, is the absence of any grains older than ca. 600 Ma in this sample. Nonetheless, a whole-rock, Sm-Nd depleted, mantle model age (Tdm) of 1000 Ma considerably older than the age of the igneous protolith strongly suggests the involvement of older (possibly Grenville age-equivalent) crust in the generation of this rock.

Motts Gneiss, Uchee Terrane

Sample TH-03 is a sample of Motts Gneiss, a quartzofeldspathic gneiss collected ∼1 km west of type locality at Stroud, Alabama (Figs. 2 and 3E). Precise U-Pb isotopic data were not readily acquired from this sample because of the very high degrees of discordance and high concentrations of common Pb in the zircons. These problems are attributed to the markedly high U contents (up to ∼5000 ppm) that characterize new mineral growth as both whole grains and overgrowths. The imprecise data available do suggest, however, that the Motts is a late Paleozoic intrusive rock. Limited data from cores to these younger grains suggest significant interaction with (or melting of) ca. 600 Ma crust. In addition, Sm-Nd analysis of a separate sample of Motts Gneiss collected at White's Creek yielded a Tdm value of 820 Ma. The antiquity of this model age compared with the crystallization age of the Motts Gneiss further supports a crustal origin for this unit, and may indicate a mixture of Mesoproterozoic and Neoproterozoic crust.

Hollis Quartzite, Pine Mountain Terrane

We performed U-Pb analysis of metamorphic rutile from a sample of the Hollis Quartzite to provide additional constraints on timing of Paleozoic tectonometamorphic development in rocks of the Pine Mountain terrane. Our objective is to compare and contrast tectonothermal development between the Pine Mountain terrane (footwall block) and the overlying Uchee terrane (hanging wall), in the area of Alabama and west Georgia where the Bartletts Ferry/Goat Rock fault zones mark their boundary. Muscovite and K-feldspar dates of 287–277 Ma and ca. 260 Ma, respectively, help to constrain the lower temperature parts of the Pine Mountain cooling path (blocking temperatures of ∼350–250 °C, respectively), but problems with extraneous argon in hornblendes (blocking temperature of ∼500 °C) prohibited gleaning meaningful thermochronologic information on the higher temperature parts of the trajectory (Steltenpohl and Kunk, 1993). Rutile, an accessory phase in the Hollis Quartzite, has a U-Pb closure temperature of ∼400–450 °C (Mezger et al., 1989), making it useful for this purpose. Red rutile grains were separated from sample H-1 (Fig. 2). Three rutile grains analyzed for U-Pb isotopes via TIMS (thermal ionization mass spectrometer) exhibited very low Pb contents (<10 ppm) and yielded nearly concordant 206Pb/238U ages of 291–294 Ma (Fig. 4D). One of these is essentially concordant at 294 ± 5 Ma (2 sigma). One rutile grain had a higher Pb content (∼290 ppm) and an older 206Pb/238U age of 392 Ma. Attempts to date sphene grains from the same Hollis sample, and thereby to obtain a metamorphic age corresponding to the closure temperature of sphene (∼600 °C, Tucker et al., 1987; Heaman and Parrish, 1991), were unsuccessful, because all Pb in the sphene was found to be common (zero-age) Pb.

These schistosity-forming metamorphic rutiles are interpreted to record isotopic closure through the ∼450 °C isotherm at ca. 295 Ma while cooling from an earlier Paleozoic amphibolite-facies metamorphic event. The rutile date is consistent with the 287 to 277 Ma 40Ar/39Ar muscovite dates (Steltenpohl and Kunk, 1993), thus placing constraints on the higher temperature part of the Pine Mountain terrane temperature-time (T-t) path (Fig. 5). The precise timing of the metamorphic peak, unfortunately, remains elusive. Deposition of the Hollis Quartzite had to postdate 831 Ma based on detrital zircons within it (Steltenpohl et al., 2004) and, as described above, the suggested Cambro-Ordovician age for the Pine Mountain Group is speculative. 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 40Ar/39Ar 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.

DISCUSSION

Peri-Gondwanan or Gondwanan origin for the Uchee Terrane

Our 620 to 640 Ma dates are clearly correlative with Pan-African and/or Trans-Brasiliano elements and strongly suggest an exotic peri-Gondwanan or Gondwanan origin for the Uchee terrane protoliths. The Uchee terrane, therefore, is likely to be another arc infrastructure terrane component of the Carolina zone of Hibbard et al. (2002). The plutonic, volcanic, and volcaniclastic protoliths imply that the Uchee terrane may comprise parts of both the plutonic root and the cogenetic supracrustal edifice to this arc. Because many of the zircons we dated are schistosity- or gneissosity-forming minerals (i.e., S0/S1 in Figs. 3B, 3C, and 3F), the upper amphibolite-facies metamorphic imprint, M1, occurred some time after the Neoproterozoic (see Fig. 5). The Neoproterozoic relics in the Uchee terrane likely had developed outboard of Laurentia, across the Iapetus ocean, along the Gondwanan margin or one of its peripheral island arcs, prior to formation of the eastern Laurentian, early Paleozoic, passive margin (Fig. 5).

Compared to other ca. 600 Ma infrastructural terranes within the Carolina zone, the Uchee terrane is among some of the oldest and appears most similar to the Savannah River terrane (Maher et al., 1991; Dennis et al., 2004). The Savannah River terrane mainly comprises migmatitic gneiss, paragneiss, and schist with pods of mafic and ultramafic rock in the core of a broad foliation warp. It was constructed and metamorphosed to upper amphibolite facies at 620 Ma, distinctly earlier than all other Carolina zone terranes. It also escaped any other metamorphic overprint until the Alleghanian, when it, too, was intruded by granitoidal plutons (Maher et al., 1991). The Savannah River terrane is geographically close to the Uchee terrane and is similarly bounded partly by the major Alleghanian Modoc shear zone (Maher et al., 1991). The suprastructural Milledgeville terrane, however, lies between the Uchee and Savannah River terranes. Unfortunately, the Milledgeville terrane has not been mapped in detail, and we are not aware of any structural or geochronological data from it (see also Hibbard et al., 2002). Geologic maps of Georgia (Crickmay, 1933; Pickering, 1976) only indicate that rocks underlying the area of the Milledgeville terrane comprise upper greenschist and/or lower amphibolite-facies phyllite, schist, and quartzite.

If the Uchee and Savannah River terranes are correlative, then the Milledgeville might be its stratigraphic cover, and, together, they would compose a major terrane internal to the Carolina zone. Its boundary with the Pine Mountain and Charlotte terranes, that is, the Bartletts Ferry/Goat Rock and Modoc fault zones, respectively, might mark a suture between two volcanic arcs of different ages. This is conjectural, of course, without geological and geochronological information that better constrains the evolution of the Milledgeville and its boundary zones with adjacent terranes. The age range for Uchee terrane zircons (620–640 Ma) may alternatively suggest a link to the Gondwanan Suwannee terrane (Figs. 1 and 5). Granodiorites intruded the Suwannee terrane between 600 and 625 Ma (Heatherington et al., 1993), but little is known about the country rocks for these plutons (Guthrie and Raymond, 1992; Steltenpohl et al., 1995). In addition, the Suwannee contains a suite of ca. 550 Ma plutons and coeval felsic to basaltic-andesitic volcanics (Dallmeyer, 1987; Heatherington et al., 1996), which have not yet been detected within the Uchee terrane. The Paleozoic portion of the T-t trajectory as presently constrained for the Suwannee terrane is ∼30 m.y. older than that for its Uchee foot-wall terrane for cooling below biotite closure (∼300 °C) (Fig. 5; Steltenpohl et al., 1995). This is consistent with subsequent extension and southward down-dropping of the Suwannee terrane from higher crustal levels as is documented by major Mesozoic rift basins along the northern edge of the terrane (see Chowns and Williams, 1983, and Guthrie and Raymond, 1992, and references therein).

Paleozoic (Appalachian) Evolution of the Uchee Terrane

Taken as a whole, our U-Pb zircon data (Fig. 4E) indicate two principal age populations in Uchee terrane rocks—one is Neoproterozoic (ca. 620 Ma) and the other is Carboniferous (ca. 300 Ma). Although more dates are needed to verify this “end-member” age distribution, the lack of intermediate “Appalachian” ages (e.g., Acadian or Taconian) is conspicuous, and combined with reported field and fabric observations, suggests a two-stage evolution: stage one—Neoproterozoic construction of the Uchee arc; and stage two—Carboniferous accretion, “Alleghanian” amphibolite-facies metamorphism, deformation, and plutonism.

The time of “peak” metamorphism associated with stage two is recorded by the ca. 300 Ma overgrowths on zircons in all rocks we have analyzed from the Uchee terrane (Fig. 5). This timing is compatible with 295 to 288 Ma 40Ar/39Ar hornblende cooling dates and documents upper amphibolite-facies metamorphism and deep tectonic burial (∼35 km; Chalokwu, 1989) of the Uchee terrane contemporaneous with the Alleghanian orogeny recorded in the foreland (Steltenpohl and Kunk, 1993). A host of Uchee terrane structures, including the Goat Rock/Bartletts Ferry fault zone mylonites and S2 and L2 fabrics (Figs. 3 and 5), formed within this time span (Steltenpohl and Kunk, 1993; Steltenpohl et al., 1992). Subsequent to metamorphism, rocks of the Uchee terrane cooled from ∼780 to ∼300 °C (zircon U-Pb and biotite 40Ar/39Ar blocking temperatures, respectively; Mezger and Krogstad, 1997; Harrison et al., 1985) between ca. 300 and 276 Ma, implying a very high rate of uplift. Using the thermobarometrically constrained average metamorphic field gradient of 35 °C/km (Chalokwu and Steltenpohl, 1989) yields an unrealistically high uplift rate (5.9 cm/yr). The significance of this rapid uplift is not yet clear. Steltenpohl and Kunk (1993) interpreted petrologic, mineral cooling, and structural data to indicate that following tectonic thickening, the once structurally higher Inner Piedmont terrane was brought down along oblique, right- and normal-slip faults flanking the northwest margin of the combined Pine Mountain and Uchee terranes (Kish, 1988; Schamel et al., 1980; Sears et al., 1981a; Steltenpohl 1988, 1990; Keefer, 1992; Babaie et al., 1991; Hadizadeh et al., 1991), tectonically unroofing the younger, underlying Alleghanian metamorphic core. This interpretation is consistent with either late Alleghanian (Permian) gravitational collapse (Maher, 1987; Steltenpohl et al., 1992) or early Mesozoic rifting.

Although a precise U-Pb date on zircons from the Motts Gneiss was not determined, the suggestion of a late Paleozoic age is consistent with field relations and existing mineral cooling dates supporting that it and the younger suite of Hospilika Granite likely are Alleghanian intrusives. Although additional studies are needed to more tightly constrain the emplacement age of these late-stage intrusives, they appear to support the assertion that high-grade Alleghanian metamorphism and associated felsic plutonism extended well into the southernmost exposures of the Appalachians in Alabama. The Uchee terrane, thus, contains a rich and diverse history of Alleghanian high-grade metamorphism, deformation, and plutonism that we are only beginning to understand.

Suturing of the Pine Mountain and Uchee Terrane

Confirmation of peri-Gondwanan rocks of the Carolina zone in direct contact with Grenville basement and its stratigraphic cover of the Pine Mountain window in Alabama and Georgia (Figs. 1 and 2) is significant. The Bartletts Ferry/Goat Rock fault zone marks this juxtaposition, and it appears to be the only place in the southern Appalachians where Carolina zone rocks contact Laurentian crust rather than demonstrably “suspect” terranes. Hatcher and Zeitz (1980) referred to the position of the tectonic boundary between Laurentia and the peri-Gondwanan terranes as the “central Piedmont suture,” recognizing that the original suture locally was overprinted by later Paleozoic tectonothermal affects. West (1998) interpreted this boundary to be the late Paleozoic suture, whereas Hibbard et al. (2002) argued that the original suture formed earlier and was excised by later shearing.

Our results demonstrate that the central Piedmont suture in Alabama and west Georgia is the Bartletts Ferry/Goat Rock fault zone. In central Georgia, we suggest that the Bartletts Ferry/Goat Rock fault zone likely terminates or merges with the Modoc fault zone, and/or perhaps other faults (e.g., Dean Creek fault; Hooper and Hatcher, 1988), as we depict in Figures 1 and 2. Figure 5 compares the temperature-time (T-t) evolution of the Uchee (hanging wall block) and Pine Mountain (footwall block) terranes across this fault zone along the Chattahoochee River, which marks the state line between Georgia and Alabama in Figures 1 and 2. The ca. 295 Ma U-Pb date on rutile from the Hollis Quartzite corroborates Steltenpohl and Kunk's (1993) suggestion that, like the Uchee terrane, the Pine Mountain terrane had occupied middle-crustal levels (amphibolite-facies conditions; see Fig. 5) during the Alleghanian event. The rutile dates also are consistent with reported field (Steltenpohl, 1988) and isotopic data (Steltenpohl et al., 1992; Steltenpohl and Kunk, 1993) indicating synmetamorphic juxtaposition of the Uchee and Pine Mountain terranes along the Bartletts Ferry/Goat Rock fault zone. P-T-t trajectories for the two terranes merge, within analytical uncertainty, at ca. 295 Ma, and thereafter they followed similar paths (Fig. 5). Thus, the two terranes were joined either prior to or during the ca. 300 Ma metamorphic event and then were uplifted and cooled as a more or less coherent block (Steltenpohl and Kunk, 1993).

It now is clear that the Pine Mountain and Uchee terranes in Alabama and west Georgia are part of a potentially large, amphibolite-facies, Alleghanian tectonothermal zone. The western margin of this Alleghanian tectonothermal zone appears to coincide with the western margin of the Pine Mountain window, where the Towaliga and associated faults have down-dropped Piedmont rocks toward the west (Steltenpohl and Kunk, 1993; see cross sections in Figs. 1 and 2). It appears likely that this same Alleghanian thermal and deformational zone continues southward beneath the Coastal Plain at least 30 km where the Alleghanian Suwannee suture is interpreted to occur (Higgins and Zeitz, 1983; Horton et al., 1984; Guthrie and Raymond, 1992; Hatcher et al., 2006; see Fig. 1). We are not aware of any modern thermochronological data constraining the along-strike boundary to this Alleghanian tectonothermal zone northeast of the Alabama-Georgia state line. It may merge with the well-documented Alleghanian zone in the east Georgia and South Carolina Piedmont (i.e., Savannah River and Dreher Shoals terranes), which projects roughly along strike of the Uchee trend within the hanging-wall block to the Modoc fault zone (Dallmeyer et al., 1986; Secor et al., 1986; West et al., 1995).

CONCLUSIONS AND DIRECTIONS FOR FUTURE STUDIES

The Uchee terrane of Alabama and west Georgia contains 620 to 640 Ma zircons that indicate incorporation of Pan-African and/or Trans-Brasiliano elements and support an exotic peri-Gondwanan or Gondwanan origin. This Neoproterozoic crust constitutes a sizable area in the southernmost exposed part of the Carolina zone, and it is among the oldest infrastructural terranes within the zone (i.e., Savannah River).

The U-Pb data document amphibolite-facies metamorphism and mid-crustal–level tectonic burial of the combined Uchee (hanging wall) and Pine Mountain (footwall) terranes contemporaneous with the Alleghanian orogeny recorded in the foreland. The two terranes were joined by ca. 295 Ma and then were uplifted and cooled as a somewhat coherent block. The northwest margin of this “high-grade,” Alleghanian tectonothermal zone partly coincides with the Towaliga fault along the west flank of the Pine Mountain window, but the lack of 40Ar/39Ar mineral cooling data from the northeast terminus of the window in central Georgia leaves an incomplete picture of its full geographic extent.

There is no obvious suggestion in the U-Pb data for pre-Alleghanian, Appalachian events (i.e., Taconian or Acadian) having affected rocks of the Uchee terrane. Combined with initial fabric studies, this information allows for the following scenario: (a) Neoproterozoic relics in the Uchee terrane developed prior to formation of the eastern Laurentian, early Paleozoic, passive margin across the Iapetus ocean either along the Gondwanan margin or peripheral to it; (b) the Uchee was rafted across Iapetus while the Taconic and Acadian orogenies were taking place along the Laurentian margin; and (c) it finally docked with Laurentia during the Alleghanian event (prior to ca. 295 Ma). On the other hand, the four samples we analyzed are not a robust statistical sampling, and more work may be needed to recognize evidence for middle Paleozoic Appalachian activity.

Our report brings up many new questions and emphasizes the fact that much work remains to be done on the Uchee and Pine Mountain terranes. The Uchee terrane needs to be systematically mapped to determine how relations documented in Alabama and westernmost Georgia carry out into its northeastern extent as well as its adjacent terranes. It is particularly critical to understand how the Uchee terrane relates to the Milledgeville and Savannah River terranes and how they all fit into the larger picture of Carolina zone evolution. The time of the “peak” of metamorphism in the Pine Mountain cover rocks remains largely unconstrained, although it is key to placing a maximum age on docking of the Carolina zone. High-precision, U-Pb and 40Ar/39Ar thermochronological data also are sorely needed from rocks marking the northeast terminus of the Pine Mountain window to establish timing of formation of major Appalachian fault zones (i.e., Towaliga, Goat Rock, and Modoc zone) and their interaction with the Box Ankle fault zone. In addition to being a suspect for the Carolina zone suture, the Box Ankle fault zone has also been interpreted as an exposed segment of the “Appalachian décollement” (Schamel et al., 1980; Sears et al., 1981a; Nelson et al., 1987; Higgins et al., 1988; Steltenpohl and Kunk, 1993; Steltenpohl and Moore, 1988; Steltenpohl et al., 1992; West et al., 1995) that had passed above an autochthonous/parautochthonous Pine Mountain terrane. McBride et al. (2005) recently resynthesized and reinterpreted seismic data to provide evidence for scattered and weak subhorizontal reflectors beneath the Pine Mountain window, but the controversy remains unresolved. It is critical to understand how surface faults exposed around the Pine Mountain window relate to the suture and the décollement. Toward this end, new analysis of aeromagnetic and gravity data penetrating the thin veneer of Coastal Plain units in the southeastern USA indicates an anastomosing network of mylonite zones in the subsurface that extends to and appears to merge with the Suwannee suture imaged only 30 km south of the onlap boundary (Hatcher et al., 2006). The aggregate width of the Carolina zone along the South Carolina and North Carolina state line is roughly 450 km, but it drastically narrows to less than 30 km in the area where we are working in Alabama and Georgia. Furthermore, the new aeromagnetic maps indicate complete excision of the Carolina zone only a few tens of kilometers to the west (Fig. 1). Future work in this area, therefore, is crucial for establishing a lithosphere-scale cross section relating the surface faults to both the Carolina zone and Suwannee sutures and to the master décollement as well.